[Vedran]

Find AR2100.bcs with NearInfinity, and add something like this:
IF
OnCreation()
THEN
CreateCreature("AAAAAAAA",[xxxx.yyyy],zz)
END

AAAAAAAA = Name of the creature (filename, to be more precise).
xxxx and yyyy = location where the creature should appear (first check if it is unoccupied)
zz = which way the creature is headed. I think it works clock-like - 1 to 12.

Or:
Find AR2100.ARE, click edit and scroll down until you see actors. This is a list of creatures in the area. Simply add another one (copy/paste existing one and edit it).