Hello, my name is XXXXX XXXXX X'X here to answer any of your electrical questions.
You could only do it if i is installed in the appropriate wiring method. Since it is a cabin I am assuming there is no attic?
Yes, there is an attic, but not accessible due to amount of insulation and design. That is, the wires to the existing light is not accessible via the attic. Cannot run the wires in a conduit on outside of wall? It is a straight run for about 20-25 feet in each direction.
Yes, if it is ran in conduit, that is fine
How is best way to connect the new wires to the wires on the existing light?
Sorry for taking so long to respond. Ok as far as the connections go, you will use wire nuts to put the wires together. as far as being able to connect the pipe you will need what is called an extension box to attach the new pipe to. However if it is not a straight shot to where you want it you would need a pipe bender. If all of this is going to be indoors, and or under a eve, I would suggest using mc wire which is a flexible metal covered wire that you can bend as you need it.
One last thing: will I need to mount the base of the existing light to a box to make the wiring connections easier? Can I run the mc wire through the box? Thanks.
Yes you will need to use a box, and connect the connector from the mc to the box. All connections must be contained within a box.