Can you attach a Signature Ability to a Universal spec tree? Universal spec trees can be added at the same price as if it were an in-career spec. Are they treated as if they were in-career for all purposes?

No they aren't.

I'm AFB ATM, but as I recall there's verbiage in the description of Universal specs that says while you don't pay the extra +10 XP to buy the tree, they don't count as career specs.

I suspect the reason they put that in was specifically for restricting Signature Abilities from being attached to them.

EDIT: Here we go, Edge of the Empire Core Rulebook, page 275:

"[Force-Sensitive Exile] is a universal specialization. The experience point cost to take a universal specialization is always ten times the total number of specializations would possess with this new specialization. While this means it costs the same as a career specialization, universal specializations are not career specializations."

What Absol197 said. Signature abilities can only be attached to a character's primary career specializations.

It can be attached to any specs in the PCs Career.

Any spec in the originak career which means its possible to buy two, although its possible I may have missed a restriction on the last, but last time I looked I couldnt find it.

It can be attached to any specs in the PCs Career.

You're right, and I should have been more clear. I added a "s" on the end of "specialization" for clarity.

Any spec in the originak career which means its possible to buy two, although its possible I may have missed a restriction on the last, but last time I looked I couldnt find it.

I think I read somewhere or heard somewhere the devs said that it is possible, you just can't swap it from one to the other. I might be hallucina-remembering, but I'm pretty sure it was addressed.

The limit is one Signature Ability per career specialization. The limit is there to encourage people to buy their way through multiple in-career specs. I don't have any of my career books with me at the moment, but I'm positive of this.

Any spec in the originak career which means its possible to buy two, although its possible I may have missed a restriction on the last, but last time I looked I couldnt find it.

I think I read somewhere or heard somewhere the devs said that it is possible, you just can't swap it from one to the other. I might be hallucina-remembering, but I'm pretty sure it was addressed.

Your insights serve you well. I had asked Sam that a year or so ago. He confirmed that you can purchase both SigAbils as long as you have multiple trees and correct talents to attach them to.