No, the PIX is only a PPTP server. Even if it was a PPTP client, what you're trying to do (similar to a LAN-to-LAN IPSec tunnel) is not possible with PPTP.
Version 6.3 of the PIX code, due out in the next two weeks or so, allows PPTP with PAT, so you won't need a valid global address for each of your internal machines, they'll be able to all use the one address.