My friends and I are starting a MERP campaign and only I have experience with MERP. One of my friends wanted to be a shape-changer, but my Second Edition MERP book does not cover how to use Martial Arts, Adrenal Defense, or Shape Change, so I am at a complete loss.
The skills you've mentioned are all in Rolemaster (in its various versions), which is what MERP is based on.
Option 1: So the (not easiest) way to do this would be to run Rolemaster instead of MERP (they're compatible, but Rolemaster is like a 6-volume encyclopedia to a single-volume dictionary of MERP!) - This is what I did for my MERP campaign. It has all these skills as well as a much larger set of options (how much money do you have?) and characters and so on; if you choose this option, ask for more details!
Option 2: Somewhat less effort!. Create a custom race for the Beorning, base in on "Men" and then add some special skills for them to use in the new categories.
The three skills translated from Rolemaster to MERP are:
Adrenal Defense: Special Skill. No bonus except for rank. Each rank gives a direct bonus to DB if the character is aware of the attack. First ten ranks give +5 each, next ten give +2 each etc. Miscellaneous Skill.
Martial Arts: Combat Skill, Strength adds, use tooth and claw when in bear form, use grappling and unbalancing otherwise. Frankly this skill should be there for any character anyway to allow fisticuffs!
Shape Change (Control Lycanthropy in Rolemaster) Taken from RMC2; use the table on MT-2 (p 77 to resolve) use the "General" column to determine if the change is successful, then adjust modifiers as you feel; -30 for daylight, +20 for full moon, etc. This skill is based on Constitution and Self Discipline in Rolemaster, so for MERP, Constitution is a good fit to add to it.