In order to get more talents for your sicarios, you may use other sicarios that are not so valuable - just fuse them to use their stars and get a new talent!
However, once you've used a sicario to get a talent for another one, you will not see it in your inventory.
The maximum number of additional talents you can assign to a sicario is two (so, your sicarios can have not more than three talents in total).
Don't panic if you accidentally closed the game while searching talents! You will be able to continue searching from the point where you stopped.