You can edit the library_prefs.xml by hand if you don't want to or can't use Poser's library to do that. But you'll have to do that while Poser is not running, as it writes to that file when closing, so Python scripts inside Poser cannot change the added runtimes without talking to Poser the same they as the library does, if they don't want to crash Poser in order for Poser to keep the changes. I don't think any of the Python libraries talk to Poser like Poser's library but rather use the Python methods - that's why they don't have undo, too.