VPN network address overlap

[Craig Findlay <craig_(_at_)_estech_(_dot_)_com_(_dot_)_au>]

I am using OpenVPN for a network to network VPN between 2 offices.
Both offices use the same private network address. This creates a 
problem when using a VPN as there is an overlap with some host 
addresses, and also a routing issue.

Both offices use OpenBSD firewalls.

Both offices already use a nat rule in pf to provide internet access to 
all the hosts on their respective networks.

Is it possible to configure pf so that one office "sees" the other 
through the VPN tunnel as being on a different network address?

For example, assume both offices use a network address of 192.168.1.0/24

If officeA has a PC with host address 192.168.1.20 then it would be 
ideal if officeB would be able to see that host as say 192.168.2.20.

Then there would be no host address overlap and we could easily setup 
the routing in the firewalls.

I have looked at the pf faq but I can't see an easy answer.

Thanks for any help you can give.
Craig