Why do you need it on steam? What is the problem with buying the game from frontier - so they get all the money and steam doesn't take it's (30%?) cut.
if it's a case of having it all on the same list, and to use the steam overview while playing (which I like to chat to steam friends), then the best solution would to simply add the launcher to steam as a "non-steam" game. You then run the launcher via steam as you do any other game, and then when that loads you run the game. Voila steam overlay should be available