Most LED lights are not direct 120 volt and they usually have a transformer to step down to the right voltage. If you splice in more lights and change the number of lights on the output side of the transformer you may exceed the amperage rating and create a fire hazard. However, if you just splice in more appropriately sized wire to fill a gap in the string, it should not cause a problem.

Bruce

Thank you so much for your prompt reply. I plan to just splice in more wire without any lights.

Then the only thing you have to be concerned about is making the splices properly and water-tight.

I believe that there are devices for low voltage landscape lighting that should work for you.