Yes, it’s available. You can follow my work at https://github.com/gschlager/discourse/tree/phpbb3.
I assume you will be able to figure this out on your own, since you want to add support for custom BBCodes. I won’t give any support until my work is finished and merged. Expect some heavy refactoring on my part which might break things for you. 
Two hints to get you started:
- https://github.com/gschlager/discourse/blob/phpbb3/script/import_scripts/phpbb3/support/bbcode/xml_to_markdown.rb
- And lots of specs in https://github.com/gschlager/discourse/blob/phpbb3/spec/script/import_scripts/phpbb3/support/bbcode/xml_to_markdown_spec.rb
The import from phpBB 3.2 works with a few caveats right now:
- Poll votes aren’t imported.
- Internal links aren’t rewritten to Discourse URLs.
- The
[attachment]BBCode doesn’t work.