Is there a good way to pull additional wire into a conduit with wires existing? Less than 25 feet in length 2 90 degree sweeps 1 1/2" conduit with 3 large wires exist, probably #6, don't know for sure but plenty of space for the 2 #12 I would like to add.

Sure, with power off, Push fish tape in, attach wires and pull in, 2 90's is not 2 bad.
If the wires are free enough, you can pull a wire out, then pull in same size conductor to replace one being pulled out, plus your extra conductors, Use a wire Lubricant as well.

Thanks for the quick reply.
I found another way to solve my problem.
I am setting up a powerline network. I can use one of the other legs of the service, and
Then use a separate fused circuit. These devices don't like circuit breakers either. I thought I was going to have to pull new conductors from the adjacent building that is connected by the conduit.
Thanks again.

OoooooooK!

Transformers are bad things to go between so just like in X-10 type systems, you will probably need a phase coupler. I'm assuming your talking about Powerline Ethernet. I'm not sure if the X-10 phase couplers such as this one Hardwired SignaLinc Phase Coupler (Signal Bridge) will work.

Before their were couplers, you had to make sure you were on Same Phase. One way is to plug long extension cord where one device will go, then go to where other device and find outlet that reads 0 Volts between Cord Hot and other receptacle Hot. If you measure 240 Volts, you are on opposite phase.