Can I add a triple crank to my old signet centurion road bike? I have an old signet centurion 12 speed road bike. The bike is in very good shape.  I'm older now and want to start riding again without buying a new bike.   I would like to add lower gears to deal with the hills around me.  It current has a 170mm 52/42 custom (brand name) crankset.  Can I easily add a triple crankset to this bike to get the gearing I want?  Does the front derailleur also need to be changed?
You probably could, but it is not clear if you should. You could get some compact (50-34) or even subcompact (e.g. 46-30) double crankset. That looks like a better option to me. A different front derailleur might but does not have to be necessary depending on what you can put up with. Witj a triple you almost certainly need a new front derailleur (although again not completely strictly) and most likely the shifter although a friction shifter could stay.
With a six speed freewheel you do not have that much choicein the back. You can try to get some Megarange freewheel 14-34 or similar. It has the biggest sprocket (granny gear (much larger than the other ones).
Modern cassettes with large ranges would require much bigger changes including a new wheel (just changing the hub is unlikely to be worth it) and widening the rear frame spacing (there is some leeway with steel).