Replace new record with own function in lookup window

I have a popup menu for creating new records for different document types in a table. How can I get the new record popup menu function in a lookup window?

I found in the documentation that I can extend the following forms:

svy_nav_fr_buttonbar_lookup_window
svy_nav_fr_lookup_window

with:

svy_nav_fr_buttonbar_lookup_window_custom
svy_nav_fr_lookup_window_custom

Is it possible to do that, if I extend the lookup window?