Dear Mario,
Sorry for the late reply.
Moreover, we are sorry to say that there have no pre-built option or addon for adding manual time.
However, we added a new option in the FoodBook v3.9.7 to add a note about the time slot to aware your customer. Please check out the screenshots below as a reference.
https://prnt.sc/13h1p33
https://prnt.sc/13h1w1f
It will help you to get rid of that situation.
Best Regards,
ThemeLooks Support Team