A monstermaker can make any entity I think, although HL might be uphappy if you asked it to begat another monstermaker
Make the monstermaker "cyclic", monster type "ammo_9mm", and target it after one second with the multimanager. Put the multimanager's own name in its list of things to target, with a delay of 90s. Now the multimanager will repeat itself every 90s (you may need to check a "repeatable" flag or something) and when it does, spawn the ammo.
CS thing? Pretty sure not.