You can't add your own characters to Articulate's Content Library.
But it's always been possible to insert custom characters as pictures. I've done that a lot with cut-out characters from https://elearningart.com/. There are other companies that provide characters. And, yes, it's helpful, because the Content Library is limited.
As I've said in other comments, the more specific one's request is for certain character traits, the more likely it is that filling the request will require licensing characters from a company that focuses on that service, buying images from a stock photo agency, or even paying for a custom photo shoot.
The Rise Scenario block does present a problem. However, I once saw Tom Kuhlmann demonstrate how to replace the scenario character with custom characters. The trick is to publish the course, and then replace each character PNG in the assets folder with a custom-character PNG. The new images must use the exact names as the original assets. Not a fun way to get new characters, but it does work.