Hi Eitan,
Pradeep's link points you to a class that can help reparent your table. However, depending on the version of ServiceNow you are running and the type of table structure you have, it may not work correctly. Be aware that running the code suggested could make your table inoperable and cause errors in other related parts of the system.
If this is a new table, or one with very little data, it's better to create a new table with the extension specified at creation time. This is why the UI doesn't allow you to reparent after creating the table, and why you see that error message when you try to force it.
It's also worth noting that the actions of that class aren't captured in Update Sets, and it's possible that your scoped application, when exported, won't have the necessary configuration information for that table to be an extension of Task when the app is installed on other instances.
Just be aware of the potential for harm before you try it.
