You surely can reconfigure the airplane to use the main cargo door on the left, using the supplied airplane configuration editor.
It might be a bit tricky to set the correct variable names in the custom XML expression, since PMDG use custom variables for all their doors but, if you cannot figure them out, you can simply choose to ignore the door status.
I'm not sure about the front nose door. It should be possible to create a configuration for it, but it might result in strange approach paths.
Note that, the total number of cargo loaders will always be 2.