Converters' main action is converting complete system, your request is much more complicated because you don't need users, polls etc... also you need some topics which converters not allow us to choose import part(s).
If you edit converter to just convert topics / messages it may harm your database because of duplicated IDs, or some of your topics will be changed with old topics.
Suggestion: Convert you whole site into new (untouched) SMF database then (you need to have knowledge about phpmyadmin / sql tables etc...) then clean topics which you are not going to use,
or you can clean your old phpbb forum (i mean deleting topics which you are not going to use), then convert to SMF (don't forget untouched new db - not your running db). Then you need to change all topic/message IDs...
This is bit tricky and contains some risks too.