If I was doing this I would not bootstrap the ftUndergro ability. I would setup the archetype to allow the selection of Favored Terrains with upgrades at the correct level.
Then you have two options:
1) Allow selecting of all Favored Terrains but have an ability on the archetype do a pre-req check for ftUndergro. If not found show an error that selecting ftUndergro is required.
2) Change the cFav2Expr field to only allow the selection of thingid.ftUndergro.
Option 1 is preferred because it allows a gamer/group to get around the requirement of Underground as the only option if they wish. Option 2 is more restrictive and does not allow for a DM to override the selection if they wish.
Then to do "upgrades" you want to work with a Resource. Specifically resTerrUpg. That allows upgrades of +2 to be given when you want.
Trying to "force" logic around the Favored Terrain/Enemy logic in HL is not going to work really well.
