What you are proposing is something like I am doing. I, too, have a bunch of DC only locomotives, and a few DCC with sound. My layout is set up with blocks for DC running, and so I can leave a loco (or loco with train) in a block that is turned off, and have the rest of the layout running on DCC. You
must not have a DC motor on DCC, or you will burn out the DC motor! There are a couple of ways you can do this:
One way is to connect the main bus for the layout to an MRC Tech 6 6.0 power pack/controller. This pack can be selected for either DC or DCC with the press of a button. I have one, and it would be the complete answer...if I could walk and chew gum at the same time!
But, after nearly frying a DC motor with the Tech 6 in DCC mode, I decided to set up a DPDT toggle switch on my control panel. The center contacts are wired to the track bus (which is broken up in to the individual blocks each of which has a DPDT switch that is wired like a DPST Center Off...one set isn't connected). One set of the side contacts are wired to the Tech 6. The other side is wired to a separate MRC 260 DC power pack, which has plenty of juice for two or three Athearn Blue Box diesels. The toggle is labelled and set so that when it is to the left, it is DC; to the right is DCC. (Be sure to check which contacts on the back of the switch corresponds to which direction the toggle is thrown. Some switches have the contacts on the
opposite side of the switch, and some on the same side. Check with an ohmmeter!) There can be other solutions, but, to me this is the simplest and "safest" for my motors.