I've seen a few posts asking about this but no simple answers, so here's the easiest way.
Lets use doorhenge's sf2 screenpack as an example. Here we have the roster you want to use along with the car bonus, which we will make "invisible" or in other words, unselectable.

in my screenpack system.def, under [select info] I currently have
rows = 3
columns = 6
and my select.def looks like this
Spoiler, click to toggle visibilty
Streetfighter/(sf2)ryu/sf2'ryu, stages/ryubg.def, order=1
Randomselect
Streetfighter/(sf2)blanka/sf2'blanka, stages/blankabg.def, order=1
Streetfighter/(sf2)guile/sf2'guile, stages/guilebg.def, order=1
Streetfighter/(sf2)bison/sf2_bison, stages/bison.def, order=3
Streetfighter/(sf2)Balrog/balrogsf2, stages/balrog.def, order=4
Streetfighter/(sf2)ryu/sf2'ken, stages/kenbg.def, order=1
Streetfighter/(sf2)chunli/sf2chunli, stages/chunlibg.def, order=1
Streetfighter/(sf2)zangief/sf2'zangief, stages/zangiefbg.def, order=1
Randomselect
Streetfighter/(sf2)sagat/sf2'sagat, stages/sagat.def, order=5
Streetfighter/(sf2)vega/sf2'vega, stages/vega.def, order=6
Bonus_Games/car2/car2, stages/carbg2.def, order=2
the problem obviously is that currently the bonus game can be selected as a character.
To fix this problem, edit your system.def so that under [select info] you now have
rows = 3
columns = 12
also in the system.def make sure you have
showemptyboxes = 0
moveoveremptyboxes = 0
now simply adjust your select.def accordingly.
Spoiler, click to toggle visibilty
;Row 1
Streetfighter/(sf2)ryu/sf2'ryu, stages/ryubg.def, order=1
Randomselect
Streetfighter/(sf2)blanka/sf2'blanka, stages/blankabg.def, order=1
Streetfighter/(sf2)guile/sf2'guile, stages/guilebg.def, order=1
Streetfighter/(sf2)bison/sf2_bison, stages/bison.def, order=3
Streetfighter/(sf2)Balrog/balrogsf2, stages/balrog.def, order=4
-/
-/
-/
-/
-/
-/
;Row 2
Streetfighter/(sf2)ryu/sf2'ken, stages/kenbg.def, order=1
Streetfighter/(sf2)chunli/sf2chunli, stages/chunlibg.def, order=1
Streetfighter/(sf2)zangief/sf2'zangief, stages/zangiefbg.def, order=1
Randomselect
Streetfighter/(sf2)sagat/sf2'sagat, stages/sagat.def, order=5
Streetfighter/(sf2)vega/sf2'vega, stages/vega.def, order=6
-/
-/
-/
-/
-/
-/
;Row 3
-/
-/
-/
-/
-/
-/
-/
-/
-/
-/
-/
Bonus_Games/car2/car2, stages/carbg2.def, order=2
what this does is a) move the car bonus to a point on the screen that your cursor can't reach and b) insert enough blank spaces "-/" so that it moves it offscreen alogether. Simple eh?

For your own choice of screenpack the technique is exactly the same. Add an extra row and make use of blank spaces to shunt your characters offscreen.
Excellent for boss characters, essential for bonus games. I Hope you find this useful.
*before you mention it, I know my use of the randomselect could make the car still being selected a possibility. That's something you can't get around. Though I find with a small enough roster or with an "accurate" screenpack that never uses it anyway such as SF2 (mine only has the randomselects while I search for characters to fill them), you don't really need to worry about using it.