As you said, you can transfer up to four generations at a time from FamilySearch to Ancestry to extend end-of-line individuals. However, you may only add one individual at a time from Ancestry to FamilySearch using the Ancestry interface. I have heard that third-party tools, such as Ancestral Quest allow you to move data and individuals more easily, but I do not have experience with that.
Please remember that your Ancestry tree is yours to play with as much as you like, but there is only one Family Tree, which we all share. If you had the ability to transfer a large number of new individuals directly into Family Tree, it is highly likely that you would create multiple duplicate records. That is why we are encouraged to add only one person at a time to Family Tree, checking for duplicates every step of the way.
I have been spending the majority of my time attaching sources to individuals in Family Tree, because sources help us to prove those people existed, and they typically show family relationships that we need to add people to Family Tree.
Dana Repouille, Plattsmouth, Nebraska