[UPDATE 27-3-2012] - IBM now has an officially supported tool: http://www-01.ibm.com/support/docview.wss?uid=swg21459627.
This is a utility that will take multiple single user installs and migrate them to a multi-user install. The general premise is that each user's Notes data directory is stored on his/her home drive (such as h:) and this will move the files to the appropriate %UserProfile% folders to make it work with the multi-user installation. I did this specifically for our own Windows Terminal Services servers, but there is no reason this shouldn't work for shared workstations, too. Just as long as Notes was originally set up a single user installation and each user had their data directory stored elsewhere, this should make it easier to migrate them to a multi-user install.
Here is the [EDIT - link removed 11/28/06]
Here are the VB5 runtimes if you need them.
I have tested this in my own environment and it worked well, but your mileage may vary. If you get any errors or need any help either post a comment or e-mail me and I'll do my best to help. Also if there are any features you would like to see added let me know. If there is enough interest I would also be willing to release the source code.