I recently repurposed a pair of clip-on earrings into a necklace & took photos of the process to share...

in addition to the earrings I bought a chain & jump rings at michaels

remove the clip-on component using a pair of needle nose pliers

sand down any raised or rough area that is exposed when the clip-on is removed

for the long piece to connect the two earrings I used part of a pin back

I cut off the pin & catch at either end

then sanded down the edges so they were nice & smooth

next I used e-6000 glue {be warned its stinky & toxic} to secure the pin back to the earrings

after it was dry I used more glue to add the jump rings - a stretched out paper clip provided support while it dried

finally I removed one link from the chain so I had two ends to attach to the jump rings & there you go...a necklace!
**edit to add: soldering the pin back & jump rings onto the earrings would be a more secure way to construct this but I don't know how yet. if you use glue, double check you've used enough on the jump rings especially & just keep in mind when you wear it that it is more delicate than soldered type jewelry.