I think you may have forgotten to post the link?
Regarding the forums, what format is your users database already in. For example, do you have some sort of CMS functionality where you have captured user's data, do you have an Excel spreadsheet, Facebook fan page etc.? In other words, how is your user data currently stored?
EDIT - sorry, this was too long to go in the comment field
Data is stored in a mysql database when a user register
So it's some sort of custom form which captures the user info into a database? If you have less than a couple of thousand users and you only need to do this once, probably the easiest thing to do would be to export your users out from the database into a CSV file - see How to output MySQL query results in CSV format? or just use PHPMyAdmin depending on your level of file access.
Many open-source forum softwares (e.g. PHPBB and Phorum) have the ability to import a CSV file, so all you'd have to do then is get a template for your import CSV file, and copy your user data with the correct formatting into this. MyBB (mentioned by Damien) seems fine, and already has some merge functionality - see http://www.mybb.com/downloads/merge-system - I'm not sure if this would suit you though since you haven't identified an existing software (i.e. you might need to write a custom importer).
One trick I have learned is to create a user in any given database driven software (e.g. PHPBB), and export the appropriate user table to CSV using the technique I just mentioned (or using the built in functions in PHPMyAdmin). You can then use this as the template, and then read the data back ito the database, see Import CSV to MySQL . This can be a hit-and-miss technique depending on how many dependant tables there are for the user table.