sub-menu function wanted on Nokia 5110 + U8Glib

I'm working on a project using U8glib with nokia 5110. Took standard menu sketch from library example and it's pretty nice.

Need also sub-menu functions, and i guess somebody already faced that situation. I know there's a separate relative library of U8glib for interactive grafics, but seems it will complicate my project too much as both the sketch size and RAM used now already close to max size.
Any suggestions would be appreciated.