I don't believe you're going to like this answer, but what I did was I created a fresh database in RM and loaded it with TreeShare. I then merged my other RM database (created from a GEDCOM from MyHeritage) into that Treeshare enable database, merged as many as possible doing RM duplicate/merge, then started the process of syncing those changes back to Ancestry using TreeShare.
I started in July, as of now I'm almost through the H's.
To connect a RM database to a tree on Ancestry you can merge the downloaded Ancestry tree into the current RM database. Make sure to clean up the Place list before you begin the merge to limit duplicate facts. It's a time consuming process but its the only way to get these two separate trees together.
Anytime you drag a drop someone from a database that is connected to a tree on Ancestry the connection to the tree will be retained, if there was no other tree connection prior. So you could in theory take one person from the downloaded Ancestry tree to your current RM database. Then open TreeShare and it will be connected to that tree. You would have to manually link each person in the RM database to someone in the Ancestry Tree. This will work if where there are not to many unconnected people. I can get TreeShare to work with 1,500 in the database. I can't do this with my 15,000 database. I do not know the point it becomes an issue, but it's something you can consider.
You can always use Compare Files to make one the master database. We have a video that demonstrates that feature.
What's New in RootsMagic 7
00:29:05 – Compare Files