Can one theoreticaly smuggle contraband with time travel?

It depends on your go to authority on the mode of time travel. In the Terminator movies, only living tissue could transport, or stuff surrounded by living tissue. By that scenario, whatever you smuggle would have to fit in your whazzou.

I would think if you could travel back and forth in time you wouldn’t need to take anything with you.

Depends which science fiction author you are reading.

One of them allowed you to travel and take things with you. A different one (maybe Maria Doria Russell??) made the time traveler arrive naked. – as in, the clothing couldn’t make the trip.

The timejumpers in Callahan’s Crosstime Saloon take @Pachy‘s statement to a whole new level as some can pop between back to their native time and return to when/where they were instantly. On their return they would have to be in exactly the same place, but they could bring stuff with them.

Also, time didn’t advance, so they could get shot, pop back to their home, get medical attention, heal up, and return to the exact same moment only minus the bullet wound. I see no reason they couldn’t pop back home, grab a gun, and pop back armed.

So theoretically it’s possible. We don’t know for certain how (or even if) time travel works and therefore cannot disprove that it’s possible.