Is there a way to turn on team fortify on a map in which the designer doesn't have it turned on by default? I looked when I was creating a couple of boards, but I didn't see an option for this.

The Board Designer can enable Team Unit Transfer (under Team Settings, and perhaps should be called Team Fortify but that's just semantics), but a game Host cannot enable the Rule if it is disabled.   Only customizeable option the Host may be allowed to change is Fog setting.

That's kind of a bummer, IMO. Team games are often more fun if you can have a little bit of interaction with your teammates on the gameboard. Shame the default is set to have no team transfer.