“using the mapping with relative ISDataSource by CustomObjects for the SchemaType, where the CustomObjects contains all the fields of table (Events, Category, Resource) but also other custom fields.”

There should be no problem using ISDataSource with custom object.

“in the CustomObject relative to Event table, I want to add a custom field that refer to Customer object, I need to add in the EditingForm a WebCombo that binded to a table that contains a list of Customers (like a Category).”

This scenario can be implemented using the extensibility feature of WebScheduler. I enclosed WebScheduler white paper that contains detail information about the extensibility feature. You can also check our extensibility sample (EventOrganizer.aspx and CarRental.aspx) at WebSchedulerSamples project.

Hope this helps.