Acquiring assets requires two things:
- 3 xp
- Narrative context
For example, you can't just "learn" to be a Gunslinger out of the blue, you also must frame it fictionally: the character trains hard by herself with a gun, or is taught by someone else, your call — it just has to make sense inside the story.
If I were you, I'd write specific conditions for buying the asset, like "this asset can only be chosen when you gather information about X", or something along these lines.
Good luck!