It's possible with H18, you just need to create the bones at the OBJ level instead. There's actually more learning material for that right now. See the “OBJECT LEVEL RIGGING” section at https://www.sidefx.com/learn/characters/
Having said that, yeah, I recommend going to 18.5 and learning the new KineFX system as well if you don't have an actual requirement to use H18. The object level rigging system will eventually be deprecated.