They could, they would just need to change the frontend. Right now they just append {"community": "old_community"} the json_metadata field of the post. Whereas the newer communities, e.g. all the ones with hive-XXXXXX in the name, set the community by using that community as the parent_author but leaving parent_permlink blank (or else it's a comment). Which to be honest doesn't seem as intuitive, but makes sense as the newer communities are accounts that have membership fields and roles etc.