Move Mailboxes from Exchange 2007 to Exchange 2010 Hosting Server

I have a bunch of exchange mailboxes on a very old Exchange 2007 server.  The server was setup using a type of hosting, so the mailboxes are from different companies.

We have installed, in a new domain on a new server, Exchange 2010 in Hosting Mode.  Becuase the mail boxes are in a different domain, I cannot use the built in MoveMailbox options.  I fear the only way I can do this is to export the mailboxes to a pst file, then create a new mailbox on the new server, and import the pst files.

I believe there are powershell commands i can use to do this in bulk?  Perhaps someone could help with these?

Or better still, does anyone have any better suggestions on how I can move these mailboxes? With less manual work, there are a lot of database, and some pretty huge mailboxes.

Thanks for any and all advice.