Replace a double ultegra crankset for a triple ultegra crankset. Is that possible?
Most people go the other way. Many new bikes now only come with double chain rings. And compact crank sets give almost the same gear range as triples.

But it should be possible. The distance the front derailleur travels is about the same. You may need to adjust your limit screws though. And of course you need to change the front shifter to a 3 speed version.