Hi, yeah this should be able to work.
At the end of your intro mission file, type this before the exit code;
End1=true
this is just a variable to make End1 true within the game.
Then create a trigger with type End1 and type End1 in the condition field. That should make it only end after the intro is completed.
Hope that works, it may not because ArmA Is different then OFP, but it should..
At Best,
NightJay