If I start taking propecia before hair transplant, will it quicken process or at least slow down shock loss?

Propecia can help avoid shock hair loss.  In general it is not realistic to start Propecia on the same day of surgery or few days before surgery, but it is better than nothing.  Of note, you do not need to take Propecia to have surgery.  It is more about diminishing the risks of shock hair loss.

I routinely recommend the use of Propecia and or laser light therapy for 3 months prior to transplant for any patient on whom I feel there is either: 1. a risk of shock loss due to susceptibility of these miniaturized hairs, or 2. a chance that these existing hairs will as a result of the Propecia or laser get stronger, and thus change the amount of grafts that may be necessary.  In either case, we are seeking to get the best possible long-term outcome.

This is an excellent drug to stop shock loss in most young patients. It may even reverse some of the hair loss that you experienced. I strongly recommend that you take this drug as recommended by your doctor