Yes.
Just do like for example, you copy the demon.xml file and name it: demon1.xml
In the XML file, you leave the name the same, but you change the looktype. Then, in monsters.xml you add the file BUT, you'll put it like this:
<monster name="Demon1" file="demon1.xml"/>
To summon the monster, you must do it by "demon1". And thats it x)