You should carefully remove 1/2 inch of the wire insulation about 1 inch back from the stripped end of your wire. Wrap the stripped end of the wire you want to add around the 1/2 inch of bare copper on the other wire and solder then add shrink tubing over the soldered joint. Crimp and seal the the bare end of your original wire as usual.
This is how we added jumpers inside connectors. One wire per crimp and other wire added by surgical cuts and soldering on the original wires, but placed away from the crimp. Not a quick job, but a reliable one.
This is how we added jumpers inside connectors. One wire per crimp and other wire added by surgical cuts and soldering on the original wires, but placed away from the crimp. Not a quick job, but a reliable one.