If I have understood correctly, there is no easy way to do that.
I think your
FH Project records everyone in your family, but
Ancestry has only your grandparents and their ancestors, possibly in four separate trees.
It is complex enough to export a
GEDCOM from
FH that has full details for ancestors of your grandparents, but privatised details for their descendants, and migrate it to a
new Ancestry Family Tree online.
BUT I suspect that is NOT what you want.
I think you want to
MERGE those missing privatised descendants onto your existing
Ancestry Family Tree and that is NOT possible without migrating via
Roots Magic using its merge features and its synchronise with Ancestry tools.
See
Parallel Family Historian and Ancestry trees (16318).
The only straightforward way is to manually add your parents, you & your spouse, your children, and your grandchildren.
I am not sure why you need to add those descendants to
Ancestry to record the
DNA results.
Why not add the
DNA results directly to your
FH Project?
BTW: What exactly do you mean by
privatised? No names? No BMD facts or dates? Or what?
If I have misunderstood, then let me know.