Exchange 5.5 Upgrade Help!!

Ok we have a new Exchange 2003 Forest lets call it sitea.forest.com we also have a child domain which is now siteb.forest.com. Heres the problem we have 2 email servers 1 in sitea.forest.com and 1 in siteb.forest.com sitea has an ADC in to the network and has acquried and set up the placeholder accounts for the exchange 5.5 users BUT I need a selection (about 50 accounts) to be moved on to siteb. Is there an easy way to do this as at present we need to keep the exchange 5.5 network operational as there are servers in there that have not been upgraded (mainly out MTA). We are in a disaster recovery state at present as our main exchange 5.5 server has gone down and although partially restored we need to get the mail across in to our new Exchange 2003 network.

Cheers

James