[horde] aditional custom hooks...is it possible?

[alex]

 there,
 
I'm trying to develop a mail system with qmail style aliasing,  managed
through horde.
I've got the perl script that does it quite fine. I only need it to hook it
up with the imp prefs, namely 
Alias_addr.
 
This is my idea:
 
User puts an alias in the alias_addr field in the imp prefs.
The alias_addr is hooked up with the function _imp_hook_alias_addr in
hooks.php (registered as $conf['hooks']['alias_addr'] = true in imp's
conf.php)
This function checks the pair $UserID, alias against the alias stored in a
database.
If it is available stores/updates the $UserID alias with the new one and
makes the exec() call to the perl script, otherwise returns an error.
 
My question is, can I make a custom hook like this one? Or is it impossible?
 
So far I've been unsucessfull. 
I don't know If the horde framework allows it, or if im doing it
uncorrectly.
 
Thanks in advance,
 
Alex