osTicket is a great ticketing system and there is no way to import customer information as there is no way to natively manage any type of customer list. While there are a lot of functions built-in to osTicket, they all surround the single purpose of generating tickets, assigning those tickets to a dept of staff member and keeping the customer abreast of changes to that ticket.
If you want to import anything to a vanilla osTicket installation you'd be using phpmyadmin or any other mysql tool to do the import. Again though, there's not even a customer table in osticket, so a lot of work would need to be done to customize your system if you wanted that functionality.
