All good answers, especially Julien's answer about how AC current is tricked into being seen as DC by a DC motor. The E-Z Command will run one DC engine while you're running DCC engines at the same time. You will be able to control the speed of the DC engine by selecting zero on the controller and that of the DCC engines by selecting their number on the controller.
It sounded for a bit like you were asking if you could have a DC powerpack and a DCC controller both hooked up to the tracks and active at the same time. For anyone else who may be thinking of doing this - Don't. The DC current will feed back into the DCC controller and there's a good chance you could destroy the innards in a few minutes. Once you hook up a DCC controller, only use the DCC controller for power supply to the tracks.
The E-Z Command is a little different than other DCC systems since it has a plug on the back that allows you to use a Bachmann standard DC powerpack to give you speed and direction control of a DC engine without having to select it. You can pick one up on e-bay for around $20, and it might be a worthwhile investment since you are planning on running DC engines.
HO Scale Modeler