While others have already mentioned fabricating claims, you, since you have an Emperor-level title, have a much better way to conquer territory (which isn't applicable to the pope but otherwise much more effective): invite claimants to kingdoms to your court, land them with single counties and press their claims. Since they have that county, they will be your vassal and will remain so once they have their kingdom.
I personally don't like having king-level vassals though because it makes the vassal territories map look horrible (Since I can't create all kingdoms because of culture, meaning that parts of the realm will be single blocs and the rest chaos. I rather have patchwork all over.), so I usually look for a way to destroy the title (either by marrying into the family and inheriting it myself or by making him revolt, which isn't that hard because kings want to be independent pretty fast).
Edit:
Go to your advisers, the chancellor has a "Fabricate Claim" mission. Set him into the county you want the claim on. It will take quite a while, depending on how good your chancellor is and cost money at the end (it scales with how rich you are, IIRC), but your ruler will get a non-inheritable strong claim on the county. If you have a chancellor with at least 15 diplo he can also fabricate a claim on an entire duchy. This however takes twice as long and is twice as pricy, but usually still worth it.