In terms of conversions, a Wych would seem like an obvious start.
1) If you're using the Glaive, then you could start with one of the Hellion weapons (I believe one of them is holding it with just one hand).
2) If you then cut off one of the blades and replace it with the blade of a Venom Blade or Power Sword, I think it will look a lot more like the Succubus' Glaive.
3) You could cut off the other blade as well and maybe have something more decorational there instead.
IIRC the Hellion weapon is held in the right hand, so this would leave the left hand free for holding an Agoniser or such.
In terms of making it look more Kabal-y, I think the only differences between Wyches and Kabalites (aside from weapons) is that Kabalites have two shoulder-pads instead of one, and slightly more armour on one leg. Hence, if you want a Kabalite-looking Succubus, you could use Kabalite legs instead of Wych legs and find or sculpt the missing shoulder-pad.
Coven is trickier, though you could give her the Wrack Agoniser arm, rather than a Wych or Kabalite one. Maybe add some of the vials/syrings bits to her (which could also represent Combat Drugs or even Phial Bouquet) or some of the mini-arms from the Talos/Cronos kit. You could even use a Wrack as the base model, rather than a Wych or Kabalite.
Hope this helps.