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The existing newBB forum built into XOOPS is loosely based on phpBB. There is a hack available (phpBB-to-newBB) that will migrate the users from your phpBB forum over to the newBB forum.
The reason I mention this is because you can use this hack to migrate the user accounts into Xoops, then all you have to do is disable the newBB forum. Works pretty good actually. There may be some missing contact information, and the avatars don't carry over, but if you're just interested in importing the user accounts for site access, it will work quite nicely. You could do as mentioned above and install the phpBB port and replace newBB, but that's your call.
As far as linking to an outside forum.... Depends on your theme and module setup. If you're using the default theme and navigation, you'll need to create a block with a hyperlink. Some themes have the links hard-coded into them, just edit the code with the appropriate URL. There are also custom navigation modules available that replace the default system module. iMenu is especially useful for this purpose.
The iMenu module is available in the main sourceforge repository @
http://sourceforge.net/project/showfiles.php?group_id=41586&package_id=105497You can download the phpBB-to-newBB hack here:
http://www.herve-thouzard.com/modules/mydownloads/singlefile.php?cid=7&lid=62Hope this helps.