Mixing ingredients while stretch and fold

that your dough isn't rising well, or that your starter isn't strong enough? 

Tell us about your starter (flour types, hydration, age, how long it takes to double at what temperature, how long it takes to peak, etc) and include pictures if you have some.

For your dough, what formula are you using, and what temperatures are you working at?  There are quite a few popular authors and recipes that show fermenting / proofing times based on proofer or room temperatures in the high 80's F, so your times are going to be much longer at lower temps.

As for your original question, adding in more starter during the stretch-and-fold can be done, but it'll take a bit more effort and folds to get it fully incorporated, it will change the overall hydration, and it may not help much with speeding up fermenting / proofing if it really isn't all that strong.  You'd be better off spreading out the dough, and spreading on a paste with a bit of instant yeast dissolved in a bit of water, and then kneading it in to be fully incorporated.  Better yet, include the yeast at the same time as you put in the original starter. 

If you really aren't sure about your starter, another option is to do a hybrid (starter / commercial yeast) where you create a commercial yeast poolish or biga, and then mix it in to the final dough along with your starter.

Hopefully you can give us more details so that someone can give you some more direct and less vague advice!

just work it in.  If it takes too long and tomorrow still not risen, work in some instant yeast softened in a spoon or two of water.