Re: Adding own functionality
Yes, as Jan said it can definitly be another db if you need it to be that way or prefer it that way. If you decide to put your tables in the same db as mojoportal you could also use a different prefix to distinguish your tables from mojoportal tables. All the mojoportal tables and procedures start with mp_ so if you give yours a different prefix it will make it clear which tables are not part of mojoportal.
Hope that helps.