The Eula: How custom can it get?

Hey there! I'm getting ready to do a BYOB for the Eula, and say I wanted to add a zippered pocket to the outside back panel--would something like that be doable? If so, how do I incorporate a request like that into my order? I'm just wanting somewhere to stash my keys on teh outside without having to "riiiiiiiiip" open the velcro every time. Thanks :)