From the PFSENSE site I read that it's "Multiple WAN Support" and also
that it's based on m0n0wall.

Then, my question is: What could trouble trying to backport that
feature to m0nowall?

I really know little thing about FreeBSD, so that's why I ask is it
would be possible to backport this functionality or it's somehow
impossible.

Thanks for your attention!
Marc.