From what I can gather Bruce, you can't do what you are saying without a small mixer of some kind.
Here is how I solved the same issue
Basically I used an old Korg Pandora by Toneworks as a small mixer, to mix the guitar input (centre) with an aux input from the headphone output from my computer (right) and this is mixed in the pandora and gives the resulting mixed output to my headphones (left).
If you wanted to use the output from your amp, you would have to take a cable from your amps output to the mixer.
In my case I dont use the amp at all for practicing as the Pandora has Reverb , Delay, compressor etc , if I need any for the guitar signal. I simply take the guitar cable from the volume pedal and take it to the Pandora.
Any small mixer would work the same way, and there are hundreds of such mixers available for little money.
The Pandora is made for guitar, and does away with the need for an amp and for practicing I find it very useful. Others don't even know you are there, even if you are playing with a full band at a good volume through your Headphones.
One final point is I use a set of infra red cordless Headphones which are reasonably cheap to buy and then I have no cables trailing around which can foul the strings. You can see the output goes to the transmitter directly above the Pandora in the first picture and I can sit at my steel independent of any wires.
This system works well for me, as I can mix the music signal with my Guitar, making either louder or quieter whilst adding effects to the guitar signal.
The Pandora has a lot more capabilities as well which I won't go into now.
Good Luck