We are hosting a benefit match that has a match T-Shirt and Hat for sale when you register however, most of the participants did not select the additional fees under the initial registration and now can’t go back and purchase said items. How do I set it up so that they can go back and buy them?
There’s no way for them to go backward in the registration and re-add something.
You could create a standalone match for t-shirt and hat registration. Send it to everybody and they can register on that to buy hats or shirts. 
You’re just going to have to keep track of who buys on the main match and who buys on this match.