The Jamma+ has additional inputs beyond the standard Jamma. Jamma boards can be plugged into Jamma+ harnesses assuming both use 8 way joysticks. Jamma+ boards plugged into Jamma harnesses will be missing some functions, usually control inputs.
jamma + has extra controler connections(more than 3-4 buttons per player or more than 2 players) so yes you can add the extra to a standard jamma cab or not use the extra in a cab that has it and just play standard jamma boards in it.