Sure thing. May have to do a little acrylic work but nothing major. A few holes here.... A new piece of acrylic there..... And viola! Sump! Just use common sense when you re-design it. Use Gravity to your advantage and it should all work out fine.
I am going to remove my bio balls and convert my filter to a fuge too. I have had mine for 6 or so years, tho, and that is what they recommended at the time. A little acrylic work, like rusty said, but its easy. (i've built other fuges before, not hard). How old is the tank? how much rock is in it?