Theme: Pad, soldering point, via, plated-through hole
How can I save a pad as a padstack in a package to the the database?
What is quite clear: You cannot insert a package with padstacks for a 6-fold multilayer into a double-sided board without padstacks.
Best practice: If you have created a universal blank board with e. g. 6 layers, then define the padstacks and save this blank board as a template. If you then create packages with padstacks, please use this template for the package design. The package now can be exported to the database as usual.
It is always important that the names of the padstacks are identical. This is what TARGET relies on to control padstacks. Padstacks are globally defined within the project like a certain "pattern" and can be used by any soldering pad in the project.
If a padstack used in a certain part is missing in the project, the padstack will be created empty as soon as the component is imported into the project. After that, the padstack can be repaired manually. But that is not the normal procedure. In menu "Actions" under "Special functions" you will find the item "Define Vias as Padstacks".
