There is an existing batch upload functionality for lists in which establishment means can be included, described more here.
I have not experimented with it yet to see whether it will update taxa and establishment means already in a checklist, or just add new things.
EDIT: have now experimented, and it will allow updating of existing list taxa, either as the whole batch or selectively from the batch. Anything new is automatically added without further intervention. Currently there is a bug that prevents batch import of Occurrence Status (present, common, absent, etc.), but Establishment Means works as expected.