ADMT - Group migration, UPN and proxyAddresses

UPN

First and foremost, add the domain's UPNs into Domains and Trusts and check when migration is complete that the UPNs are all correct.

proxyAddresses Attribute

To remove proxyaddresses from ADMT's exclusion list:

Adam Arkwright | Technical Blog: proxyAddress Attribute doesn't copy when using Active Directory Migration Tool (ADMT) 

To migrate groups 

set ms-ds-consistencyguid on the source domain group objects:

Powershell:

$guid=(Get-ADGroup -Identity "GS_OHF Wireless Users").objectGUID

$base64 = [system.convert]::ToBase64String(([GUID]$guid).ToByteArray())

$hexstring = ([system.convert]::FromBase64String($base64) | % ToString X2) -join ' '

$binary = [byte[]] (-split (($hexstring -replace " ", "") -replace '..', '0x$& '))

Set-ADGroup -Identity "GS_OHF Wireless Users" -Replace @{"ms-ds-consistencyguid"=$binary}

Then perform the group migration (if groups are already migrated this will simply add the attribute)