Add users to a group from another Active Directory domain

Hi Folks,

I need add users in a group the active directory through the FIM 2010 R2.

My scenery it is:

Domain A with FIM 2010 R2 provisioning users for Domain B;

I need get users the Domain B and add in group in Domain C.

What's better way, create FIM portal for them, or create aditional script/development for FIM 2010 R2.

Thanks a lot!

Wilsterman Fernandes