Currently you are able to PostFailure on the FailureMessage through the document for the current transaction, but you are not able to post a warning to the document and have it show up in the Warnings dialog. It would be nice to post the warnings like overlapping walls or duplicate marks so they can be resolved later if people don't fix them right away.
https://forums.autodesk.com/t5/revit-api-forum/how-to-create-a-warning/td-p/8054281
Can't find what you're looking for? Ask the community or share your knowledge.