
Claxon |

Can you be more specific with weapon feat you are referring to?
Unchained Rogue's do have access to the Ninja Trick rogue talent of the core rogue, but do not have the Master Trick which allows the selection of Master Ninja Tricks.
So you could pick up a Ninja Trick, as long as it's not one already on the rogue list (you couldn't for instance pick Combat Trick from the Ninja list as it's already on your Rogue list, and thus would prevent you from gaining two extra feats since you can normally only take Combat Trick once).

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So, I'm assuming you want to take ninja trick to take combat trick, for a weapon proficiency feat, and still have the combat trick from their rogue side left. Sadly, it says that you can't take a ninja trick if it has the same name as a rogue trick. (I'd link the text, but I'm on my phone right now- the info is on both PFSRD and archives of nethys though, under combat trick.)
You could take ninja trick to get improved unarmed strike, which is kind of like a weapon proficiency.