Other forms: carpetbaggers
An outsider who pretends to be an insider is a carpetbagger; he's a person who tries to take advantage of a group by joining it only for his own personal benefit.
Northerners who moved south during Reconstruction in the 1860s and 70s were the original carpetbaggers, named for their suitcases. It was a derogatory term then, and it continues to be used with contempt today. You can use the noun carpetbagger to describe a politician who is running for governor in a state where he's only lived for six months.