Well since LOK is my favourite all time game series, I think this would be a great idea! I can imagine a Rahab model, maybe about the size of a Tervigon, that would be pretty awesome. Youd have to be really good at sculpting though. Melchiah would also be a kick ass looking model.
So yeah I reckon the only way these can be done would be to do a lot of green stuff sculpting, or sorting through all the armies to find pieces that would work. I'll link some pictures below so people could have a better idea of this, and also could possibly suggest viable conversions.
Pretty good picture of Rahab.
Melchiah would be the most nurgle like monster I believe.
Kain would be an amazing model. I wouldn't be surprised if someone hasn't already had a go at making a miniature of him.
Raziel would be slightly more difficult I believe, but again, if someone is good with green stuff sculpting in general, it could work pretty well. Raziel is my favourite character by far.
Dumah could look pretty cool. There is actually a figure of him, that could be worth looking at for reference.
Zephon would be quite difficult since he is mainly just a giant insect, and half his body is like a wall. But I guess you could go with the Alien queen approach that he could break off of the wall XD
And finally Turel, I think in this monster form he would be way too big to keep to scale with the others. However, the Turelhim would be awesome little models, and would fit with the others, since Turel himself wasn't seen until Defiance.