Yes + no:
* You'd want to do that in a carboy probably, as more fermentation = more yeast = you don't want that in the keg.
* If you're adding significant enough plain sugar (sucrose, rather than maltose), you may end up with undesirable levels of acetyldehide or other spoilers.
I did this once to a kit beer (before I knew better): take a bunch of honey (I think I used about a kilo), and a bunch of mangoes (I used two). Peel + blend the mangoes, and boil the hell out of the paste mixed with water. Dissolve the honey, cool, pitch into fermenter. Bumped the beer up by one or two % abv.
Beer tasted alright... definitely not something to brag about though (although I've yet to have a syrup-kit beer that is).