I took out a closet in my bathroom and had to relocate a switch to a nearby wall. When I fed to cables up through the floor behind the drywall they were not long enough to connect at the box with the cables going on to the fixture. Is there a safe way to join on more cable?
If what you are trying to move is wiring for light then yes it is quite simple to join extra wire to the existing length.
It is only in low voltage applications that increasing the wiring length will have a noticable affect on the circuit, in higher voltages it still has a theoritical affect ( see Ohms Law) but it would be negligable.
There are however some rules to follow:
1. Always isolate the power to the circuit at the consumer unit
(fuse box) then test that it is dead (you may get the wrong
2. make sure the additional wire is the same size and type
3. When connecting the wires ensure no strands of wire escape
4. Make the connection waterproof
As for the connector itself ask at the DIY store as there are several methods and devices.