you could follow the "Show all posts" link on your profile and scrape those pages...
Does Phorum show the complete text of each post? If not, it would take days to pick up each individual one. In php/bb you can set how much to display, and if your posts are short you might really get the whole thing to appear in the Show All page. That's in Search (using yourself as author), not in the Profile.
:: detour to investigate ::
WooHoo, there's even an "all available" in both 2 and 3. I've never used it, but good to know. If Phorum has the same option, you're all set.
And, uhm, I really don't think anyone here is going to tell you how to scrape the pages. But that's OK, because two minutes after posting the "but how?" query you had a "D'oh!" moment as you realized how easy it is.