Seems this can be done through
EasyApache in the WHM setup (may require root access, or permissions granted from the root owner), and would be VPS/Semi-Dedicated/Dedicated.
Simply load up the wizard on the
EasyApache (Apache Update) in the
Web Host Manager (WHM) for your server.
Much of the wizard is a matter of replicating existing settings, but when you get to
6. Exhaustive Options List, then add the
mbstring to be installed (tick box), finish the rest of your settings, then save it. Backup everything. The Build the profile you've created, wait for it to finish, and hopefully you've got a working compile.
Doing it through cPanel and WHM at least allows you to get there help with support. I'd suggest you make a note of what versions of everything you were using before as well.
When I get a few minutes I'll throw this into a 'tutorial'.
(I was linked to this page for details on how it works:
http://docs.cpanel.net/twiki/bin/vie...pache3/WebHome)