Is it possible to reheat sweat joints to fill a slight leak with solder?
Adding more solder almost never works, because the problem is almost never "not enough solder" - the problem is almost always poor pipe preparation, where some part of the pipe is not cleaned and fluxed (separate things - physically clean, then chemically clean via flux - flux can't cure macroscopic dirt.) Overheating one or both parts when soldering can also cause this, by burning away the flux and oxidizing the pipe.

A properly prepared joint will wick solder all around by capillary action. An improperly prepared joint will never wet where it's improperly prepared, so it has to be taken apart and both the pipe and the fitting need to be properly cleaned and refluxed before reassembly.

Excess solder (as you might add while trying to fix a leak improperly, on the theory that if some is good, more must be better) is not benign - I have personally hunted down a pipe that had pressure when there was no flow, but almost no pressure when the valve was open; having eliminated the valve as a cause, I took the pipe apart joint by joint, and found one elbow almost filled with solder, with just a tiny hole water was making it through (that's inside the pipe, where there was supposed to be a 1/2" hole.)