I have done this sort of thing on a 3.2.5 OpenACS, and I assume that something similar will work on 4.x OACS as well.
In my case you write a small tclsh script which generates the exact INSERT statements you need. You can get the format of the statements from the user registration .tcl pages. In your case you just need to generate a file with the right info.
Use the split command to split the email addresses between the left hand side and the right hand side. LHS is the "first name" RHS is the "last name" (and drop the @ sign of course).
In the example above, "mailto:agapetvl@swbell.net"; would become "agapetvl" for the first name and "swbell.net" for the last name. The users can log in and change this later if they so choose.
