No idea. I've never given adding this feature any thought.
If it were me I would look at adding an entry to the ost_config table. (you probably want to automate this some how, maybe by writing a plugin or something)
Here's what the entry would look like.
id would be the next entry so you dont need to supply it.
namespace I would use user.# which would be username and the id of the user.
key would be something like expiration_date.
Value would be the datetime for when you want it to expire. so something like today at midnight 2020-03-26 23:59:00.
You would need to add a field to each user that you can define (datetime). And use this data to populate the ost_config table and code to display when you view a user.
then lastly you would want to figure out how you want to handle accounts that are past their expiration. Then write the routines / code to check that and handle what you want to have happen.
