Tommy,
I think the answer may be to contact Roland to see if they have the Cubase Patches for your instruments. They are in the form of XML files. If you can find the Patches, you just select add MIDI instrument, browse for the file and select it. Cubase will install the Patch.
The instrument Patches will contain all the sounds of your mks-50 and JV 1080. This makes it real easy. My Cubase came bundled with a Yamaha instrument so it has a lot of Yammie instruments included already. So depending on what instrument (manufacturer) your Cubase was bundled wiih will determines what instruments are supported.
Joe H