I am switching to crossbow, Getting old and pulling compound is hurting everything. I see some kind of damper to quite the string. can that be added after the purchase?

Depends on the bow, there are some that are easy to add and some that are not, some bows that are made for it that they sell them for and some not. What bow are you looking at?

I tried then on my Excalibur when I first got it back in 2012. The aftermarket ones would not stay on the strings. I think most crossbows made today are quieter than they were 10 yrs ago. If they are installed at the factory or at a proshop then they will do much better than anything you add to the strings to dampen the noise.

On your Excalibur, Air Breaks, string suppressors, ex-shock string dampeners added if they will work with your model bow will cut the noise in about half. I was very surprised how well they worked the first set I put on a Matrix 400 for a customer. The old beaver balls like used on trad recurve bows work on crossbow strings too and help for sure like they do on a recurve or long bow. https://excaliburcrossbow.com/acces...de/default&_tax_query_product_cat=42

Other bow companies have different noise reduction methods

A heavier bolt will be slower but also quieter and often more accurate