Hello, I'm new to this site and I'm looking for advice to hear my garage this coming winter.
I have a single story ranch with a basement that is heated with an electric hot water heater that supplies heat to the baseboards on the main floor. There is an attached garage that has a baseboard heater installed, but they are not connected to the water supply. My question is could I just connect the garage baseboard to the supply and return lines and use a ball valve to cut supply to the garage when it is not needed? The lines are already there, they are just not connected and I am worried that attaching them with throw off the balance of the system.

the big question... how are the house baseboards piped? if it is a continuous loop then you could cut the loop and extend it to the garage but you can not shut the garage heat or the loop will not function.
you can add a couple of monoflow tees to the loop and feed the garage that way. then you can shut a ball valve to stop the flow to the garage but the house will continue to heat.
option 2 is what i would choose..

You're using an electric hot water heater as a boiler? There's too many unknowns to be able to answer your question. That isn't a very efficient way to heat your house, unless you're using solar.

I would be concerned about the garage loop freezing and bursting if you turn it off.