There is a way by modifing the staticData.sqf for your mission, I've done it before and can post the code to go from when I'm home. All you have to do is find the classnames of the vehicles you want to use and insert them.

I deleted the mission but alas, Google Drive has saved the day.

You need to put this in the init.sqf of the mission:

	waitUntil {!isNil "ALiVE_STATIC_DATA_LOADED"};
 
	call compile (preprocessFileLineNumbers "staticData.sqf");

And then in the root of your mission folder make a file called staticData.sqf and put this in there:

http://pastebin.com/EEbDRt83

I've commented each section to easily distinguish what it alters.
You can add or subtract the amount of vehicles in each section (I am not sure if there is a limit?).
You can also remove certain things I believe, ex. if you dont want to edit each type of possible ammobox then just delete it out of there.

Hopefully this helps a bit.