How to Create Custom Attributes in FM2020?

Hi. I have been trying to add custom attributes to FrameMaker 2020 to try and integrate it with a CMS that uses a specific scheme for such things in its own integrated editor. Thus far, I have not found the means to do so. Ideally, I ought to be able to set a Choice in terms of value for such attributes as well. Some of my attempts are recorded below:

1. Preferences > CMS > Custom Attributes: This only supports specific CMS systems and does not seem to add those attributes to the list of attributes available to different elements.

2. Structure > DITA > Customise DITA > Customise DITA Shell: I can remove attributes from the list to hide them but not add custom attributes.

3. Insert > Conditional Tags > Create New Tag: These use a completely different notation from XML attributes.

4. Adding them to DTD and importing that to FrameMaker.

Any suggestions on how to proceed would be greatly appreciated.