When you exported the contacts, did you use the 'Account Contacts' type? That would be the best to use (it is the only type that save all data (except tags and photos). You should have a .csv file downloaded.
1) Open the .csv file in a text editor or spreadsheet application.
2) One should see a field column labelled 'fileAs'.
3) Change the value for each contact - in the 'fileAs' column - to the desired number from the following list:
(Blank) = Last, First
1 = Last, First
2 = First Last
3 = Company
4 = Last, First (Company)
5 = First Last (Company)
6 = Company (Last, First)
7 = Company (First Last)
8 = Custom - one must type: 8:<custom string>
4) Import the resulting CSV file (as 'Account Contact' type).
RELATED BUGS
Unfortunately, Zimbra doesn't currently permit users to change the default fileAs setting.
Vote for these bugs if they would help you:
Allow user to change default fileAs value:
https://bugzilla.zimbra.com/show_bug.cgi?id=8723
Allow user to add custom fileAs string (when creating a contact):
https://bugzilla.zimbra.com/show_bug.cgi?id=3218
Add fileAs strings for Nickname and Company (Department).
https://bugzilla.zimbra.com/show_bug.cgi?id=41681
LOST DATA
When exporting and importing two types of data are lost: tags and contact photos. I'm not aware of a convenient way to backup and restore contact photos, but you can preserve the tags:
When importing one can add a field/column labelled 'Category' and include tag names there (separated by commas).
When exporting one can export using 'Yahoo contacts' type - this will include contact tags (but exclude other information one may want).
As a workaround, one could export both Yahoo-type and account-type and copy to the Category column from the Yahoo-type .csv file to the account-type file.