Is this something that can be added?
No, because if we do so, then we would be put Smack in a position to include all sorts of code for non-standard compliant specifications (like custom message types (which is something you should not define)). I follow strict policy to not include code that deals with unspecified, in terms of RFCs and XEPs, behavior. Smack is build against a specification, not an implementation.
I also think that it's not a good idea to extend packet listener/collector/filter to non-Stanza elements. Everything that is not a stanza is used as stream control feature of some sort. Users using the library should not have to deal with that, it's something the library tries to abstract away from the user.
I'm curious what that kind of custom handling of the XMPP server tries to achieve. Could you elaborate that?
Although XMPP includes the term "extensible" there are certain limitations how it should be extended. See also XEP-143.