I am using an all-grain recipe for a Belgian ale. My OG came out to 1.052 rather than 1.081-5. What can I do to raise the OG before pitching the yeast? Will a pound or two of malt extract help?
You can use an extract gravity calculator like this one to help you figure out how much extract you'll need. http://brewerslair.com/index.php?p=brewhouse&d=calculators&id=cal22&u=eng
Since these calculators are normally for the total gravity of an extract brew, you need to cheat slightly. Gravity is essentially additive, so drop off the 1s (the 1 is the water, which you can disregard) and and subtract your measured gravity from your target gravity to get how many points you need to make up for. 0.083-0.052=0.031, so input 1.031 into the target gravity (remember to put the 1 from the water back on there). Then input your final volume, the type of extract, and you should have the exact amount of malt you need.
As others said yes you can boost the OG.
More importantly you need to address why the gravity was missed by a large margin. Here's a couple thoughts considering it sounds like a Belgian double or strong.
Did you forget the Belgian candy sugar?
Did your grain have the diastatic power for the mash? IE too much Crystal / roasted malts, very high water grist ratio.
Was the grain even milled? If so was cracked fine enough.
Was the grain the proper amount? Unit conversions can be tricky. :-)
Did your mash even convert? Sounds like you may have only utilized the crystal malts. Similar to 2. Use an iodine test to check if the saccarification is done.
How old was the grain? Was it stored in a cool place? If too hot it could have lost diastatic power. >149°F typical temp in a cargo container or car in the summer will denature beta-amylase.
Was the boil long enough and an open boil to allow enough evaporation for the recipe? This can dilute an OG 20%, say if it was a 5g batch and had 0 evaporation.
Hope this helps.