Official comment
This is planned for the future. We are updating the ONTRA-editor code to include a section for "Favorites" that can be individual blocks. So, you could configure a header and footer block, save them as "Favorites" and have them available from the "Add Block" function.
This will come first to the ONTRApages editor soon, and then be adopted into the ONTRAforms and ONTRAmail editors as well.