Should I import users of last year's conference?

Is it recommended to import the user data from last year's conference?

This depends very much on your event.
 
If you have mostly the same persons every year, this might be helpful.
But for most events a lot of the people (authors / participants) change every year, so it might be easier not to keep all users.

Furthermore, experience shows that users often do not remember whether they had an account or not.
They might also forget their user details / passwords.

So in most cases it causes less problems / questions when users have to register anew.

If you still decide to import users of last year's conference, please use the import function.
You can find a full manual here:
How to Import Users

When you import users, please send them a bulk e-mail with their user names and passwords.
Please note that you can only send passwords in bulk e-mails from ConfTool if the users have not edited their passwords themselves.
You can find more information here:
Is it possible to send passwords by bulk mail?.