I found these while opening the Alien Swarm files, all the beta versions of the Harvester, Parasite and Mortarbug. Perhaps you can somehow make tham into NPCs? Or another idea is because there is no beta parasite egg, probably you can make the harvesters "spawn" the parasites? Please do read and reply to this

EDIT: There is already an SNPC pack that is daym ace and contains all of Alien Swarm's creatures. (Beta and final versions)