The next in the title process for transferring them over would be "willing" them, as described by Caspian here:
Posted By Caspian at 11:45 AM - Fri Feb 24 2017
3). Willing. Because not everyone wants to play all of their titles, we're giving people the ability to will away their titles to other. This is something that can be done after domain / settlement selection. The person who owns the title chain will participate in domain / settlement selection, will pick their settlements and any domains, and then can, at their discretion give some or all of their titles over to other people. In doing so they forfeit all rights to the titles they held.
Note that this does not transfer EP, Influence, or anything else. It simply grants someone else the title and the ability to manage the settlement / domain. Think of it as though it were willing your domain / settlement to someone post-launch. It just happens now instead of post-launch so they can begin managing their domain / settlement immediately.
Willing the title over will not merge the domains together however, and will require a Sedecim to change the county land boundaries.
- The Sedecim happens every 16 weeks. This is the only opportunity you'll have to change your domain's borders without going to war or undergoing a revolution.
- The process of changing domain borders requires a vote of people at all levels involved. For example, re-drawing county lines requires a vote by the mayors/barons, sibling counts, the count themselves, and the count's duke - during the Sedecim. Re-drawing domain lines is hard, and isn't something that can be done easily or frequently.
Source: Solving Title Problems
There is no ETA on when it will be available, but I suspect it will be sometime before it will be ready.