1) Write a script that inserts the nescessary data in the database tables and the sys_datalog.
Also, need to change the "Mailbox Quota" label to "Hosting Quota"
Why do you want to change the mailbox quota label to hosting quota if it is a mailbox quota? Dont you think that this will confuse your customers if you put wrong labels on the fields?
3) Changing the way the ispconfig is intended to work means to write a complete new frontend library.