Assuming everything were to go down without a hitch, you could probably do the entire process inside of an hour, assuming you have the main forum setup wherever it is setup, all the forums are running on the same logical server, and nothing goes wrong with the server itself.
The plugins required would be
mergeSMF found here and the
SMF Admin Toolbox found here. The first will merge one board into another board, and the other will handle getting the accounts going.
mergeSMF does have warnings about the primary, recipient forum being larger than the secondary, merging board. To do this safely on a live/production server, you may want to clone the BM forum here into the new lemuria forum and then merge the M&F forum into it. Guides for restoring a SMF backup or moving a SMF forum to a different location should be available easily enough through Google.
That said, mergeSMF appears to be built for MySQL, based on the discussion, so your mileage may vary. What it does as far as topics are concerned appears to be pretty straightforward, so it could all be done as database actions if it came down to it. SMF itself has the ability to merge abandoned posts into a user's account, so you could even do that if you didn't want to import the user database.
The forums that mergeSMF merges into another, don't appear to be altered from what I understand of the plugin, they just appear to have their database copied, edited, then assimilated into the recipient forum.
