If you want to import a larger bunch of SNMP-devices in to OpsMgr you will probably go thru the disovery wizard way to many times.

Instead of walking thru that wizard every time I asked the network team to write a CSV-file with all the devices and then used this function to import them.

Function Add-SnmpDevice {
        PARAM (
                [Parameter(Mandatory=$true )][string] $FromIpAddress,
                [Parameter(Mandatory=$true )][string] $MonitoringProxy,
                [string] $ManagementServer,
                [string] $ToIpAddress = "",
                [string] $SnmpCommunityString = "public",
                [int32] $SnmpVersion = 2
        )

        # Single ip ?
        If ($ToIpAddress.Length -eq 0) {
                $ToIpAddress = $FromIpAddress
        }

        # Check SNMP version
        if ($SnmpVersion -ne 1 -and $SnmpVersion -ne 2) {
                Throw "Only SNMP version 1 and 2 supported"
        }

        Write-Host "Setting up discovery for SNMP-devices..."
        Write-Host "       From: $($FromIpAddress)"
        Write-Host "         To: $($ToIpAddress)"
        Write-Host "  Community: $($SnmpCommunityString)"
        Write-Host "   SNMP ver: $($SnmpVersion)"

        $networkDeviceClass = Get-MonitoringClass -name "System.NetworkDevice"
        $DeviceDiscoveryConfig = New-DeviceDiscoveryConfiguration -MonitoringClass $networkDeviceClass -FromIpAddress $FromIpAddress -ToIpAddress $ToIpAddress

        # Set Community String
        $encoding = New-Object System.Text.UnicodeEncoding
        $encodedCommunityString = $encoding.GetBytes($SnmpCommunityString)
        $DeviceDiscoveryConfig.ReadOnlyCommunity = [System.Convert]::ToBase64String($encodedCommunityString)

        # Set SNMP version
        $DeviceDiscoveryConfig.SnmpVersion = $SnmpVersion

        # Get management server
        If ($ManagementServer.Length -eq 0) {
                $mgmtServer = Get-RootManagementServer
        } else {
                $mgmtServer = Get-ManagementServer | Where-Object {$_.Name -eq $ManagementServer}
        }
        If ($mgmtServer -eq $null) {
                Throw "Cant find management server named $($ManagementServer)"
        } else {
                Write-Host "Found management server: $($mgmtServer.name)"
        }

        # Find proxy agent
        Write-Host "Lookup of proxy agent named $($MonitoringProxy) ..."
        $ProxyAgent = Get-Agent | Where-Object {$_.Name -eq $MonitoringProxy}
        If ($ProxyAgent -eq $null) {
                Write-Host "No agent named $($MonitoringProxy) found, checking managementservers"
                $ProxyAgent = Get-ManagementServer | Where-Object {$_.Name -eq $MonitoringProxy}
                $ProxyIsMgmtServer = $true
        } else {
                $ProxyIsMgmtServer = $false
        }

        If ($ProxyAgent -eq $null) {
                Throw "Can't find agent or managementserver named $($MonitoringProxy)"
        } else {
                Write-Host "Found $($ProxyAgent.Name)"
        }

        Write-Host "Starting discovery..."
        $DiscResults = Start-Discovery -ManagementServer: $mgmtServer -DeviceDiscoveryConfiguration: $DeviceDiscoveryConfig

        If ($DiscResults.CustomMonitoringObjects.Count -eq 0) {
                Write-Host "Cant discover any objects"
                Return 0
        } else {
                $ObjectCount = 0
                Write-Host "Found objects"
                $discresults | select-object -expandproperty CustomMonitoringObjects | Select-Object Name | Format-Table -HideTableHeaders

                $DiscResults | ForEach-Object {
                        Write-Host "Adding object to proxy..."
                        if ($ProxyIsMgmtServer -eq $true) {
                                $ProxyAgent.InsertRemotelyManagedDevices($_.CustomMonitoringObjects) | Format-Table SnmpDevice, ProxyAgentPrincipalName, ManagementGroup -HideTableHeaders
                        } else {
                                Add-RemotelyManagedDevice -proxyagent $ProxyAgent -device $_.CustomMonitoringObjects
                        }
                        $ObjectCount++
                }
                Return $ObjectCount
        }
}

And two examples on how you can use it to add some devices:

# Add a devices in the range 192.168.100.240-.254 with the community "SomeSecret"
# Use mgmtserver.snowland.demo to do the discovery and add the devices with snmpmonitor.snowland.demo as monitoring proxy
Add-SnmpDevice -FromIpAddress "192.168.100.240" -ToIpAddress "192.168.100.254" -SnmpCommunityString "SomeSecret" -ManagementServer "mgmtserver.snowland.demo" -MonitoringProxy "snmpmonitor.snowland.demo"

# Add a single SNMPv1 device with the "public" community, use the RMS to do discovery
Add-SnmpDevice -FromIpAddress "192.168.100.10" -MonitoringProxy "snmpmonitor.snowland.demo" -SnmpVersion 1

So we ended up with something like this:

Import-Csv ".\snmplist.csv" -Delimiter ";" | ForEach-Object {
        Add-SnmpDevice -FromIpAddress $_.IpFrom -ToIpAddress $_.IpTo -SnmpCommunityString $_.Community -ManagementServer $_.MgmtServer -MonitoringProxy $_.Proxy
}