At the lower end you can use the foot cradles to assist a regular bar pull up Certainly the rows will help to gain the strength to weight ratio to a point where pull ups are more feasable Try overshortening the TRX so that you are able to sit underneath the handles and pull up from here. Once you are...

Here is an indepth video explaining how to do place your heels in the TRX. Keeping even tension and pressure on the TRX while working out is essential to keeping your heels in the foot cradles while working out.