Can hair grow through concealing fibers?

No, camouflaging fibers do not block hair growth. You can use these without that concern. If you are using topical treatments, (such as minoxidil) these must be completely washed out before application of the minoxidil.

The answer to this question is generally Yes the hair can go through Toppik as mostly they just sit on the scalp.  What you want to be careful of though is using so much that they get into the hair follicles themselves and damage those.
Toppik in general should be usd sparingly, remember that even very healthy hair has some scalp visibility so don't completely cover your scalp.
Another option you might try while regrowing your hair is a henna scalp tattoo.  Remember the goal of Toppik is to reduce the contracts between the scalp and the hair so making hair thinness less apparent.
Don;t forget your ultimate goal however which should be to first stop and then reverse your hair loss and regrow your hair.
Look for a hair regrowth specialist who can identify and treat your personal root cause of hair loss and who offers a guarantee on results.