Spliced connections are very common. How they are spliced can vary from a terminal block to a simple solder connection. I can't imagine a plane where every power consumer gets a dedicated CB. The key is to understand the current draw. If you transponder draws two amps and is fed by a 24 AWG wire protected by a 5 amp CB, you should have no problem splicing in another 1 AMP device. Use proper butt connectors or solder and protect. Same with the ground. Splicing into a ground that goes to the common ground bus is just as good as going direct to that bus, assuming you know the current load on the wire you are splicing into and don't exceed it's capacity with the added load.
If you don't want to splice, just run the new wire back to the transponder CB and crimp on a ring terminal and add it to the CB output with the other one. You can also run the new ground wire to a location where you can ground to the airframe via a ring terminal and screw. Assuming it is not a device that is sensitive to ground issues.
One thing to consider with CB's is you want some form of isolation for critical devices. For example if you put a cig lighter socket on the same CB as your nav GPS, any failure/short of that cig lighter will pop the CB and take your GPS offline. Often the short condition perpetuates and CB can't be reset until the offending component is removed, which is often impossible during flight.
Just another opinion to consider.
Larry