How do I transition multiple installations to a single Multisite installation?

I have a client who runs 8 different blogs, currently on 8 separate installations of WordPress. I’m trying to encourage them to move to a single MS installation instead to make it easier (mostly on me) to maintain the different sites, themes, and mix of plug-ins. Unfortunately, I shied away from MU for the longest time, and now I have no idea where to start with MS.

Changing Plugin Load Order

I’m writing a plugin, bar, which is dependent on another plugin, foo, being activated, and I need the functions from foo to be available when bar loads. Normally plugins are loaded in alphabetical order based on the directory name, so bar is loaded before foo. If I rename the bar directory to zbar then it gets loaded last and works fine, but I’m looking for a more elegant and correct solution.

Fatal error: Call to undefined function wp_get_current_user()

I have some strange error or may be I do not have the skills to tackle this issue.
I am building a plugin for Multisite. When is use is_admin(), my plugin works fine but when I use is_super_admin it shows me this error Fatal error: Call to undefined function wp_get_current_user(). I did my search but could not be able to find any solution.