I've never tried to add IBU's after the boil, but I think you could do it. As far as I know, there are a couple of ways.
1) You could buy isomerized hop extract. This is basically the part of the hops that bitters the beer. If you drop that into your wort before or after fermentation to give it a boost in bitterness. Northern Brewer has a product called HopShot that does this.
HopShot The page also has a link to help you calculate how to use this product.
2) You could boil some hops in a small batch of water or wort to get as much of the alpha acids isomerized and then drop that into your wort. I suppose you could do this at any time, but I'd want to do it before fermentation.
Of the two options, I think the first one would be the best, because it wouldn't dilute the beer.
However, unless you forgot to add hops to your beer, or the IBU's are so low that your finished product will be sickeningly sweet, I'd consider letting it go as is. See what kind of beer you brewed and drink it. Then do a better job on the next batch.
