Sure, that's fine.
A good example of blending both methods is the nabassu in Fiendish Codex I: Hordes of the Abyss.
Okay, I'll have a go at revising the current merge with the dead.
Incidentally I'm not happy with the name and would like something better - Undead Fusion? Assimilate Lost Soul? Necromantic Gestalt? Absorb Corpse? I don't know, nothing really sings to me, although Absorb Corpse is the least objectionable.
Revising...
Merge with the Dead: If a lost soul kills a humanoid or monstrous humanoid it merges the body with its own flesh, for every victim it absorbs in this fashion the lost soul gains 1 HD, an additional claw attack, and its fast healing increases by 1 point. The victim cannot be returned to life with a
raise dead or
resurrection spell without first destroying the Lost Soul it has become part of, although it can be brought back to life with a
wish or
true resurrection (this removes the HD, attack and fast healing advancement the absorbed victim gave the lost soul).
Furthermore, two or more lost souls can merge together into a single lost soul, the Hit Dice, number of attacks and fast healing of the resulting monster equals the sum of the individual lost souls'.