Hey, I'm adding what I learned yesterday when importing, because I had the same problem. Also, it's a good idea to export what users you do have and then clear the contents except for the first row to get a good template for importing.
1: I used a formula to make usernames in excel combining first and last name. The forumla for excel is =CONCATENATE(A2,".",B2) but replace your cells with the right names. Mine was C2 and D2. I put a . in the middle because CIMY didn't like the _ in the username. Hit enter, don't click anything else or you'll add a cell or change the cell name.
2. I then clicked on the bottom right corner and dragged my select box downward through the column to create usernames for everyone in my list.
3. I then went back to the top, right clicked or control clicked if you're on a Mac on the column. I copied it. Then right clicked again to paste. A small paste block appears in the top right corner of the column. I clicked on that and said Values Only. I then saved the file.
4. When I went to import, where it says text delimiter, I deleted the double quotation mark. Then I imported the CSV file. And voila! Takes a while, but it was done!