Someone had posted a bounty for NAT over IPSec, the post is still there in the Bounty Forum on this site. I'm not sure this actually has a distinct name, but some people call it "NAT over IPSec", "Policy NAT for IPSec", etc. This functionality does not currently exist in pfSense, though with enough interest (and perhaps bounty money) it could be added.
Basically what you need to do is NAT before the traffic enters the tunnel, and/or after it leaves, like so:
Site A: 192.168.1.x <1:1 NAT> 10.0.1.x <–- IPSec ---> 10.0.2.x <1:1 NAT> 192.168.1.x :Site B
Site B uses the 10.0.1.x addresses to talk to Site A, and Site A uses the 10.0.2.x addresses to talk to site B. It will appear to each side and though the other end does not, in fact, share its same subnet.
If only one-way communications will be initiated, you only need to do NAT on the far side, so when you try to talk to the address your system will know to send it to the tunnel and not try to reach it locally.
Some have had success with bridging networks via OpenVPN, so both sides can have addresses in the same subnet, but I believe in that case they still can't conflict, just use separate areas of the same larger subnet. Someone else more familiar with this OpenVPN tactic may be of greater help in this area.