jcovert,
Jeff the best way to run your P1 in conventional with the DCS is using the Z4K and the side receiver. As the others have pointed out, all this is spelled out in Barry's DCS O Guage Companion, 2nd addition. Set up your Z4k with the side receiver and the DCS hand held remote, and as Greg points out you can even run conventional and P2/P3 engines at the same time, even on the same track, with a little planning.
Not only that, the DCS can control and run TMCC & Legacy via the DCS hand held remote also. With the addition of an AIU you can also control switches and other platform automated features, via the DCS hand held remote. With the AIU auto switching and auto route running are also an option. In short there is not a hole lot the DCS can't run. It's a matter of learning how to use it, definitely invest in the OGR DCS Video guide and Barry's DCS O Guage Companion if you want to fully educate yourself on DCS. There is a lot to learn if you want to understand how to build and operate an advanced train layout with the newest modern technology.
We try very hard to get boys and girls who like computers interested in our hobby today, once they realize how much fun the DCS really is, the kids become just as hooked on modern train hobby, as we were with our Conventional Tin Plate. Because the kids have so many more options today, its hard to get them interested initally, our hobby needs to cultivate these childrens interests if we want our hobby to continue onto the future.
PCRR/Dave