I want to add a wire to connect a security spot light to the eave of my garage roof. I have identified a 120v, 20a, 12/2 conduit in the attic that connects two outlets in the garage wall. I have done substantial testing and believe that this conduit services only those two outlets. Running the new wire from the last outlet box will be very difficult. Can I cut the 12/2 conduit in the attic where it is readily accessible, add a junction box, and re-connect the cut wiring and the new wire in the junction box? I plan to use identical 12/2 conduit for the new wire.

Hopefully you have enough slack to do this, and once cut, you may reroute a little to gain some length.
If it is pulled tight, you will have to have 2 junctions for the splices and add a short piece of cable between the 2.