i wanto to create user that have access to few sites
This wont work anyway as this FTP user wont have permissions to store files to more then one site. Its not a matter of creating the FTP user manually or with ISPConfig.
Every site has its own linux system user to separate them and your FTP user can have just the permissions for one of them at a time. This separateion is nescessary to ensure that not all your site data gets accessed by other users and that not all sites get infected in case that one site gets hacked.