The thing which is important about ram is the fast access times. doing it via USB would kinda negate this. PLus check out how much RAM your motherboard can hadle anyway, it may not like going over that limit much.
Your best bet is to just get new RAM sticks and put them on the MoBo. Using USB, at no matter what the speed, is going to be much slower than just plugging it directly into the MoBo, or using an expansion card as someone else said.