You know, you can add weight by placing a small dumbbell between your feet/ankles, you don't need a belt. Some people grab a DB between their knees, too.
In my experience, using resistance bands to learn how to do chins or pullups has been the best so far. Your gym might have bands that you can hook on the bars, or just grab your own band at
resistancebandtraining .
You can use a heavy mini band or medium band to start.