Users with extra mailbox: namespace/mailbox in userdb lookup?

[MRob]

Hello,

We have a need for some users to have an additional folder created and 
subscribed for them.

Is is possible to return an override for the default "namespace inbox" 
containing the needed mailbox definitions in the userdb lookup for such 
users? If so, how would the userdb lookup result be formatted? 
Presumably as just one long string, but how would the setting be named 
given there can't be a space in it? Can I replace the space with an 
underscore, e.g.

userdb_namespace_inbox = '{ mailbox Drafts { auto=subscribe special_use 
= \Drafts } mailbox Trash { auto=subscribe special_use = \Trash } }'

Or is there some other way to do this?