I don't plan to do anything soon. Adding subdisks to a striped or RAID-5 plex changes the address mapping. It's possible to do this, but the logistics are horrendous, especially if you want to do it on-line, and it would take a long time to do. Nothing planned at the moment. An expandable volume isn't much use if the file system on it doesn't know about the additional space.
Many people ask for expandable ufs.
This isn't really a Vinum issue, but Vinum makes it interesting. At the moment, there's nothing useful to show. It needs to find out about it. Nothing planned at the moment. It would be nice to be able to spread the data on a Vinum volume over multiple systems.