Margaret's answer covers one of the options: Only export the ancestors of one person/line when creating the GEDCOM in the other system.
As of a few days ago, we have a second option: Exclude everyone you don't want to include when importing to WikiTree.
After upload you now get a GEDMatches report. It will show all the names in your GEDCOM. You can check to skip everyone you don't want imported. To make it easier, you can use the "multiskip" options for selecting ancestors and descendants.
This would be tedious for really large trees. In those cases you'd still want to limit the export.