Internal mail flow outside without disable mailbox and user access

Hi,
it organization two domains A - Exchange 2007 and B - Exchange 2013. Migration from A to B by export/import mailbox, because all user at domain B and only mailboxes at A.
Migration plan:
1.Enable mailbox for user at B, after that only external mail will flow no new mailbox
2.Export mailbox A data to PST, and than disable A mailbox
3.Import PST mailbox to B new mailbox.
4.Sync contacts, from A to B, and from B to A
5.Clear outlook cache
How I can make (coerce) Exchange 2007 (A) internal  massage flow to external smart host for custom user, without disable mailbox and user access.

Thank you!