GMail supports retrieval of email from another server via POP3 or IMAP.  However, to validate GMail you will have to update your MX records to point to GMail until it is verified, and then re-point them back to the third party provider.  

So, I would establish a Google Apps account and purchase enough licenses for the 'internal' users, setup those mailboxes to retrieve the ISP email, update your MX records until Google Verifies, then repoint it back to the ISP and have the consultants continue as they always did.  

Just one of a few ways to accomplish what your trying to do.

Cheers,
Eric
Google Partner at FuturityCloud.com