What is the light count of the string? MOST 100 light strings are actually two 50 light strings built together (Fifty 2.5 volt lamps together in series). If you notice three wires running together, then only two wires between lamp 50 and 51, and then the three wires begin again, that is where you can break them.
And yes, as long as you keep
track of which
wire is which, you can extend them.
I've worked with twink lights waaaaaay too much.