Dynamic group based on custom security attribute

Can anyone answer this question.  Can or should i be able to create a Dynamic group filtering on a customer security attribute.

Yes I know you can filter based on extenstionattribute1-15 however i have noted that accounts create in Entra don't appear to have the option to view extension attributes plus these come from an on prem created account.

So the questions are:

1. Can I create a dynamic group using a custom security attribute and if so how because the custom attributes don't show up in the Property options when creating the dynamic group query
2. How can I add to the extension attributes for non on prem sync accounts (accounts created in Entra)