My quick-and-dirty converter for openBB worked like this:
For each user, read the data, apply the bbcode stuff to sigs, write into phpBB's user table.
Get the categories out of the openBB table and write into phpBB's category table. While doing this, create an array mapping old IDs to new ones.
For each category in turn, get the forums out and write them into phpBB's forums table, using the categories array to transform the parent categories. While doing this, create an array mapping old forum IDs to new ones.
For each forum, get the topics in it, and write them into phpBB's topics table, using the forums array to transform the parent forums. While doing this, have a cup of coffee to try to get rid of headache, and also create an array mapping old to new.
For each topic, get the posts in it, and write them into phpBB's posts and posts_text tables, running the bbcode functions on them as you go.
Finally, run the sync() code on each forum to make sure the details displayed on the front page and in the topic lists are up-to-date.
Have another coffee.
My converter doesn't destroy data at all (in fact, it'll probably get confused if you don't have a blank phpBB database when it's converting stuff