Adding ftp account, which will use given os user:
pure-pw useradd ftp_account -u system_user -d /home/home_for_ftp_account
Password change:
pure-pw passwd ftp_account
Removing ftp account:
pure-pw userdel ftp_account
Remamber, each command that change user information need to save those into proper database. You need execute this command to do it:
pure-pw mkdb