From AC17, you can only assign Building Materials (via the Fill Tool) within the Profile Manager - just like all Construction Elements.
If you've created a new fill, you need to assign it to a new Building Material and then it'll be available for Profiles.
In the Floor Plan, the Fill Tool can access any fill type - Cover, Cut, Drafting and Building Materials.
I hope that helps.
James Badcock
Graphisoft Senior Product Manager