Since I built my Khorne Demon army I've only made one addition to the army and that was a unit of OOP Blood Letters, the old beefed up metal models.  I've had this model setting on my desk for a quite a long time.
The ole Belakor model.  I thought it would look pretty cool in the army and have wanted to paint it for quite some time.  So I think it will make a cool addition.  I did not do any conversion work on it.  My old army is all mounted on smooth faux blood pool bases.  To prep the base, I sanded and shaved all of the relief off it.
Back when I built the army, I used pieces of cut up plasti-card for the bits of land, covered in HO Scale railroad ballast.  I had also added heads and demonic looking hands in the blood pools.