For Wargs (and in line with your exsisting lists), i would add it as a optional upgrade for wildling commanders, along the following lines:
+1-3 attacks(roll D6, 1-2 +1, 4-5, +2, 6 +3) (warging a weasel isnt as effective as warging a bear)
-1 command (for being notorious loners and unsociable)
and either the "ambusher" upgrade from WAA or simply a re-roll to repesent their ability to see more of the battlefield than normal, i wouldnt add to their command range as they cannot give orders via creatures they are warging into.
I'd also limit the upgrade to subordnate(wizard) lvl characters.