is it one of the aluminum benches? if so id say its not worth it. unless you coat it with about 7 feet of wax, it wont slide to well. i had one for about 2 years, and it slid pretty poorly.
buttttt, if you do go ahead and do it, you probably wont be able to weld it seeing as its aluminum. i would say your best bet is drilling though strong spots on it (on the under side of course, nothing you'll be sliding on) and then bolt on some wooden supports.