Cell frequencies are very tight. Modding the antenna would be very difficult. I have similar issues in large stores (metal framed mainly), but have always had this problem, with every GSM phone I've owned. It's pretty much a given.

Some stores (mainly tech stores, like BestBuy or the like) are becoming smart to this and placing a repeater in the center of the store (inside) with an antenna drilled through to the roof. It may be worth mentioning it at the service desk if it's a large store that frequents tech customers. They only cost about $300, and pennies to operate, which for a store that wants people to stay in their store and shop, is a small price to pay. Most store managers just don't know it's an option.