Username differences between NT & 2K

I have been the network manager at a large School in London for the past 5 years. this year we have upgraded to Win 2K. This is where the problem exists. Our NT usernames are of the order 99SurnameInitial of firstname, fine on Nt, but not allowed on 2K! We are using NTusermanager to import 270 pupils per year, this has worked fine for the last 4 years. when we tried to use the program for the 2K server, it reset polices and other file, AGHHHH! How can I use the program NTusermanager to add this years intake. We are using Win XP Pro on one site and Win 98 on the other. Help please.
Malcolm S. BA