[horde] Set "from_addr" as pre populated "enum" field

[lst_hoe02 at kwsoft.de]

Hello,

we are on the way to Horde 5 and try to get the prefs.local.php for  
Horde altered so a enum filed is used for the from_addr preference, so  
the user is only able to choose the from_addr out of a list of aliases  
obtained with a database query.

What is the easiest way to use a Database query for standard SQL  
source in prefs.local.php?

Is it correct to use $userId as logged in user?

Thanks for any help

Andreas