As bill mentioned you can purchase hop extract to correct for bitterness, but you need to be a bit careful with what you buy. Several stores including Northern Brewer sells a product called Hop Shot, but this is designed to be boiled with your wort, so you couldn't add this directly and expect to get much bitterness. I'm not sure if you could boil some in plain water for 60 minutes to isomerize the acids first or not? Maybe someone else here knows.
As another option, HopTech sells isomerized alpha acid extract that can be added after fermentation to increase bitterness. You can find it on their website. It is a bit pricey, so I guess you need to figure out if the beer is bad enough to justify the extra expense or whether to just drink it as and increase the amount of hops next time around.
If it were me, I'd either drink it as is or dump it and use it as a learning experience rather than dumping more money into a beer that you are already unhappy with when there are no guarantees that it will be better in the end.