Try this:
make a marker where you want them to respawn, named whatever you like, I'll use "here" for this example
make a trigger covering the mission area, activated by enemyside not present repeatedly
on activation [] exec "needmore.sqs"
then in a script called needmore.sqs
"unit type" createunit [getmarkerpos "here", groupalpha]
repeat that line as many times as required
at the marker area put a trigger activated by the enemy side present repeatedly
on activation [groupalpha,1] setwpppos getmarkerpos "gohere"
make another marker where you want them to go and name it gohere
that should work for you, haven't tried it, all off the top of my head. good luck!