Can I add to the wire length of C9 LED Christmas Lights?
Splicing in 20 feet (round trip) of 22 gauge wire will produce a negligible drop on the wire, depending on the current being carried. At 1 Amp, the drop at 120V is less than 1%.

The biggest challenge is making the splice weatherproof. If you do not seal the splice properly, water or snow can get in and cause issues. If both splices are affected, a dead short can happen. To minimize this, use heat shrink, electric grease, and offset the splice so that the two parts are not side by side. Keep in mind, making this wiring change may be frowned upon by your insurance if something happens.