Can led "christmas" lights that plug into 120volt household electrical outlets be used/converted to be used for low voltage outdoor landscape lighting (powered by a 12v transformer?

Sure, you would have to change some resistors, may be sets of leds in each string, would have to cut in and change resistors.
Why convert back and forth? Don't use the transformer, just plug the 120 volt lights into the 120 volt outlet the transformer would be plugged into?
They may not be rated for outdoor use?
If you wanted to switch on with the low voltage landscape lighting, then you would need a relay so when low voltage is present, it also energizes a relay, switching on 120 volt lights with your Low voltage Timer/Photocell?

I think Strat and I would both like to know more about what you had in mind...