I'm in process of migrating several business blogs into a single Multisite install, but after much research, I'm finding that there's no elegant way of getting all of my users and usermeta from one site to the other. The blogs are all heavily-populated with hundreds of posts, hundreds of users, and thousands of comments.
I know I can dump the single site tables to an SQL file, and then a lot of people seem to recommend a simple find/replace to fix the table prefixes (e.g. 'wp_users' to 'wp_4_users'), but this seems inherently dangerous, as what if I there is content in one of the tables that contains the string 'wp_'? I'm sure it's unlikely, but it just seems messy.
And then there's the whole issue of importing users, especially considering that the user id numbers on the various sites will almost certainly conflict, meaning that not only will the users need to be assigned new id numbers, but the usermeta tables will need to be updated to reflect those changes.
Given the popularity of Wordpress Multisite, I simply can not believe that there is no good solution for these piddly little issues! Is there a better way?