I added a pot of coffee to my secondary without thinking much about the water content. Now the SG is lower than I want (weak body). Could I put it back into the kettle and boil the excess water off without destroying it? Any other options?
Don't boil it.
I would not just add sugar as this will be converted straight to alcohol and thin out the body/mouth feel further. Todd's suggestion to add some maltodextrine will give you a fuller body, but without adding any alcohol as most of the longer poly saccharides are unconvertible by yeast.
I would add a little sugar and a little of the maltodextrine, to give you the ABV you wanted to account for the extra volume and add a bit of body back that you have thinned out.
Also you could bottle it, and let it carb up, as the carbonation will help a little with the perception of body and mouthfeel.
When I add coffee I normally add the grounds and let it steep for a 1-2 weeks or I add a couple of shots of espresso, both to secondary.
The last coffee/chocolate stout I did the coffee and cocoa went straight into the primary and I still got a fair amount coming through in the bottles. I did add twice as much as the recipe suggested to compensate for the CO2 carrying off the aromas. This had no secondary and went straight into bottles from the Primary.