Exchange 2003 to 2010 migration

We currently have onsite two Exchange 2003 mailservers and are in the process of migrating to Exchange 2010 that is is a data centre. We have a site 2 site vpn connection between the two and after intial issues of installing Exchange 2010 and upgrading the AD it is now working with most users moved over. We have had to recreate the connectors once where mail was not getting routed and we are monitoring queues on all servers in case this happens again.
We have a few mailboxes on the onsite servers that we want to leave for the moment, but we want outgoing emails on this server to be sent via the SMTP hub transport on the EX2K10 server in the datacentre instead of the internet connection on site.
Is there anyway to relay mails from the onsite EX server to send via the Data Center server instead of directly being sent via its own SMTP connector which will show the onsites NATed address? The main reason for doing this  is and we also use a Mail Filtering solution for incoming mails and want to use its outgoing filtering as well, but to do this emaisl need to come from the same IP as the incoming.

Hope this make sense