It's likely to be tricky. Every time your script uses any SQL, it would need to be modified for work with the flat-file. It could be done, and it you've got a nice abstrtaction layer, it'd be relatively easy. But it's more likely that you won't and it'll be difficult.
A simpler approach would be to run this script in the same database instance as your forum. You might been to mess around with table names - I generally try and prefix all table names with a short application identifier (or two), which makes it fairly easy to run multiple applications off a single database instance.