In patch 1.8 the bug that caused the AI to never add provinces to the HRE has been fixed. Under what circumstances does this happen?
I believe this bug has been resolved and from my experience, the only time the AI has added provinces to the empire has been when that particular AI nation was the emperor(i.e. when/if Austria inherits Hungary). It also makes sense that this is the case as I would never add provinces unless I was the emperor. Adding provinces adds imperial authority and if you're not the emperor when you're adding provinces, you're not getting any imperial authority. See http://www.eu4wiki.com/Holy_Roman_Empire#Imperial_authority for more info on imperial authority.
It's also worth noting I've never seen an AI nation outside of the empire join the HRE by adding their capital, without being the emperor.