you can add a formula field for entities with DateTimeRange that just returns the date parts. then you can show them on the same calendar as entities that only have DateRange
vice versa: you can use a formula that adds times (maybe midnight to 23:59) for the entities with DateRange, so they can be shown with entities with DateTimeRange
However, in both these cases, the entities will not be able to be moved on the calendar view (since the formula field is read-only and can’t be updated).
A more sophisticated solution can be done, using standard fields and automations, which would allow entities to be moved within a calendar view, but in order to make an appropriate suggestion on how, it’d be good to know how you think users will/should interact with these entities.
For example if a user changes the date of an entity on the calendar view, and this is an entity that uses DateTimeRange, what should the time values be?