If your succubus or incubus is also your Guardian Demon/ess, and they are of the Powers of Hell (their name may or may not be in the Demons list on Joy of Satan) then it is possible, if they are very close to you and you to them, and you can merge at the soul.
Either they (the succubus or incubus) or Satan does this.
It can also be undone, as I have seen, and had happen with me, as I fell out of a relationship with a Demon. We are both now on neutral and civil terms.
I am not sure how much I can talk about this, I just thought I should mention this as a possibility that no one wrote about here in this thread.