Shapechange into animal companion?
 

I'm interested in tackling converting the Lycanthrope race(s) from the "Bite Me! Guide to Playing Lycanthropes" book.

In it, werecreatures in their animal form get stats based on stats from a ranger's animal companion of the same type. This includes the progression in size, ability scores etc. that happens with animal companions somewhere around lvls 5-7.

Would this be possible to do? Or can a shapechange effect only use the rules from Beast Form spells?

Technically yes it is possible, if I am understanding correctly then yes you can recreate a activation ability script that copies and overrides the stats, size, etc while adding a if statement which checks for the minimum level to allow the changes. If this is being done without a animal companion attached to the character the problem then arises that you will probably need to rework the change shape ability to do this job for you. I don't know this book so it is hard to give you deeper advice.