One trick that some people use is to change the individual game settings in MAME so MAME accepts more than one button for a given function.
In the Input (this game) menu you can map more than one keystroke to a function.
1. Highlight the input
2. Press enter
3. Press the button you want mapped to that function
4. Repeat steps 2 and 3 as desired
If you want to reset to defaults:
1. Highlight the input
2. Press enter
3. Press escape (clears inputs)
4. Press enter
5. Press escape (restores defaults)
Your button setup:
123
456
For one button games, map all 6 player buttons to P1 Button 1 in MAME:
111
111
For 2 button games, maybe a map like this:

121
121
For 3 button games, map them like this:
123
123
For a "shift" function, I
strongly recommend against using a pinball flipper button -- it will
almost certainly cause pinball gameplay problems.
IMHO, you'd be far better off either:
1. Having a dedicated escape button on your CP
or 2. Setting MAME to exit when P1 Start and P2 Start are pushed at the same time
The only commonly used gameplay function that you don't currently have on your CP is Exit, so the "shift" is probably unnecessary once you have everything properly configured.
If you still
really want a "shift" button, consider placing it on the back/top where curious kids and drunk friends are less likely to find and press it.
For information and wiring tips to make Visual Pinball/Future Pinball flippers and MAME work together better, use BYOAC's search button up top and search on "flipper" or "magnasave" and username "PL1".
Scott