I just have a question if after you've become a Warder, can you become a Psyker later?

I'd want to argue no, as people with psychic ability are not allowed to enter the alt spec, and in a way like prestige classes from D&D, if you no longer meet the prerequisites, you should probably lose the bonuses, but that's a different game. Having said that, you are supposed to be born a psyker, and that doesn't stop other people from being exposed to the warp, and becoming a psyker, later in life; these can't all be "slipped through the cracks" cases, and something later awakened the power. You weren't a psyker then, but nothing stops you, later. They trained you, which is what Psylakis Warder, at least, is, and then you learned another ability, or were forcefully given it.

I'd say no, as the Warder isn't as good a guardian if they are, themselves, more susceptible to warp shenanigans, but I can't say the rules expressly forbid it, if you don't agree that losing a prerequisite retroactively disqualifies you from using what you got, until you are back in line with it, AND that it would cost you the trait, which itself doesn't care what you are; it's the alt rank. I'd say don't do it, to keep in line with the intent of what the job means, but RAW, I don't think the game will stop you.

I also agree with venkelos, while adding in that I really don't think being an Awakened Psyker is particularly good in this game. You only get one Psy Rating, you're supposed to take the Renegade Techniques which are not amazing, and you create nothing but a host of problems for yourself and your group. Being a Warder is so much stronger.

Personally, I'd house rule that, once you become a Warder, you can't become an unsanctioned psyker. You chose something with the prerequisite and flavor of a non-psyker, so that is what you are. Though the RAW can't explicitly forbid it, it's pretty heavily implied the two don't go together (a case of Rules as Written vs Rules as Intended I think).

As far as rolling for mutations, if you pop psyker on the mutation list, I'd just say re-roll, or spend a fate point to pick what is directly above or below psyker (I can't remember).

Just wanted to update this, and say thanks, both for agreeing that it was possible (sometimes I just love RAW stuff) but since Erathia pointed out it wasn't strong I'm not going down that path, now to find a good substitute

It is honestly stronger in a way to be a psyker through mutation than with this but if you add this to being a psyker through mutation it can be quite good.