Procmail only supports these actions:
[list=1][*]Deliver to an Mbox[*]Deliver to a Maildir[*]Forward to an email address[*]Deliver to a program[/list=1]
I don't know much about DNS, but it seems if your clients are running their own mailserver, you should be able to forward messages to internal email addresses, which would mean creating an internal MX record somehow, so you could forward
bob@foobar.com to
bob@serverb.foobar.com. Is it possible to have an MX lookup on serverb.foobar.com return something different than an MX lookup on foobar.com?
Also, why would Server A be required in your case? If the mail is for "us", then deliver the message directly in the users mail spool.
If DNS cannot be setup to accommodate the other server, I think your only option is to deliver to a program using a pipe. You can probably have an action like:
| sendmail <all switches needed to deliver to specific server>
BTW- you would probably get a better answer on the procmail mailing list. Chances are, someone there has had to do this.