If you are looking for a tool that can strip wire in the middle of a run,
then look at the following:
jokari dbp 1907711
It is not easy to find one of these but they are well worth it when you do.
These strippers will allow you to cut the insulation in the middle of the run
and carefully expose the wire underneath. The amount of wire that can
be exposed is limited by the elasticity of the insulation. However, it is
normally possible to expose enough to be able to solder another wire to
the exposed wire.