Can i take advantage of my existing 12 volt landscape lighting......

[Dooner]

....to splice into to prvide extended hours of light for egglaying and maybe heat?.....The new light(s) would be mounted inside the coop and be on a timer.

 

[Bear Foot Farm]

You might be able to add a light, but it won't be enough power to generate any heat

 

[stoopid]

X2, definately no heat. You can check the output on the control box, but if you add up all the bulbs it is rated for, you'd be surprised how low the voltage really is.