RE: Zombie Dryad's
Of course, Space Magic! Everything is possible, baring the exclusion of things that are in fact, impossible.
I imagine the Dryads of Undeath to look a bit more pallid than perhaps normal ones (Like a birch or ash coloured bark.), although obviously they could be of any colour. Perhaps with a bit of rot-bark/infestation, such as you might expect or see on a dying tree, and most likely with no leaves on them (Do the normal ones have leaves? I don't recall). Broken branches may be expected as well, though might carry a bit of mis-translation with them, as it's unusual to see a man walking with no leg, but a tree-lady without an arm might pass as 'Normal,' plus the fact they have no true 'Flesh'/'Guts' to expose, nor bones.
I suppose as an alternative they could be more like Ghouls, in the sense of cannibalistic (Or rather Eating Human flesh), or as a sort of 'Dead Collector' type theme, with body parts of various other races skewered on their branches: Arms, legs, heads, guts wrapped around, or the likes, gruesome stuff. That could work pretty well as a Druid-turned-Necromancer utilizes previously learned talents to aid in his necromantic endeavors.
I mean really, it's all about how you wanna flavor it, how you wanna make it work; But the choices are endless, essentially.
It's one of the reasons I was compelled to get into the vampire counts. Honestly, I could collect any warhammer army, and with the right tweaks, play them as VCs, plus have the pleasure of making entirely over-elaborate themed armies that are amazing.
Just imagine an Ogre-turned VC army, but since ogre's are too big base wise, the whole 'Character' base of the army could be statted on Gnoblars that struck a deal with vampires to usurp their Ogre oppressors! Oh the lawls to be had.