The obvious has already been stated: the Plant mutations already do something a lot like that.
I’m new to the game, so it may be that I’ve missed something, but, if we did want a way to get closer to the Triffids, the one thing that I would suggest adding is a Plant mutation that makes Triffids perceive you as one of their own, call it “Plant Pheromones” or something.
If that wasn’t enough and you really wanted to do one further, maybe add another mutation to make it possible for you to take control of Triffid creatures to some degree (after all, NPCs aren’t the only companions you can obtain; there are pets, livestock, robots {aren’t there?}, zombie slaves, and friendly blobs).