Not sure if they customize shirseys, but they do have a customization station. Also, everything there is more expensive than it is online if possible I'd customize it online if I were you and pay extra for shipping and I believe it'd still be cheaper.
yeah i ended up doing the online thing, but the selection sucks as to what it used to be.
on nationals.com, you could pretty much design your own shirt (front, back, sides...all different kinds of numbers, names and logos), now it's trash.