Activity Type requiring Work Item

We want to force the use of a Work Item if the Activity Type is not an "Out of Backlog" type, but there's currently no way of setting this up without having to create dedicated backlog items.

In our company we use different Activity Types to differentiate our time usage, but some of those Activity Types are dedicated for Out of Backlog activities (which also contains days off, or vacations).

A lot of peoples register their times without selecting the designed Workitem (who exist) and only use generic comments like "PBI" or "Task".