If you are migrating from Exchange 2003 to 2010, when you export the list of allowed relay devices, following this article http://support.microsoft.com/kb/935635 the output will typically be in this format:

10.1.0.1
10.1.0.2
10.1.0.3
10.1.0.6
10.1.0.7
10.1.0.8

Name the file IPList.txt
So, with the script below, you can import the list of ip addresses on your receive connector

$RecvConn = Get-ReceiveConnector "Ex2010\AllowedRelay"
Get-Content .\IPList.txt | foreach {$RecvConn.RemoteIPRanges += "$_"}
Set-ReceiveConnector "Relay Connector" -RemoteIPRanges $RecvConn.RemoteIPRanges

When you are migrating from Exchange 2007 to 2010, we use powershell command to export the list, and a powershell script to import

Here is the powershell command to export the list. Change the exchange 2007\allowedrelay part to be the correct server\receive connector name.

(Get-ReceiveConnector "exchange2007\allowedrelay").RemoteIPRanges | select Lowerbound,Upperbound,RangeFormat | sort-object Lowerbound| export-csv c:\rc.txt –NoTypeInformation

Use this script to import it onto the exchange 2010, change the second line to be the correct exchange 2010 server name \ receive connector name

$csv = "c:\rc.txt"
$rc = "EX2010\RelayConnector"
$impcsv = import-csv $csv
foreach($line in $impcsv)
{
$ipAdd = $line.LowerBound
$conn = Get-ReceiveConnector $rc
$conn.RemoteIPRanges += $ipAdd
Set-ReceiveConnector $rc -RemoteIPRanges $Conn.RemoteIPRanges
}