Yes it is possible. I have done this for zooming to an extent. I created an extension, defined a custom interface for my extension. Implemented both IExtension and my custom interfaced on it. Put the extents I wanted to zoom to as public properties on the extension (doubles.) In my other project with the command, I reference the assembly with the extension. Then from the application object, I get a reference to the custom extension running in the other ArcMap, cast it to my custom interface, set the properties (doubles.) For literal values, that is fine, if you want to pass in objects (a layer for example,) you need to use an object factory to create the object in the ArcMap process space.