I assume these time slots will be a specific service only offered on one day and other services will be offered on the other days. If that is the case, you really can't. You can block out those times from online booking and only schedule them by manually entering them. 

Another posible way is to add a staff member. If you are the only person registered to square appointments, this will make you pay the $50.00 a month charge for it. Adding a staff member, Lets call him party central! Adjust Party Centrals hours to only those particular time slots you wish to only have the slots. Make sure Party Central's avaible service is only what you call the party slot. .Make sure to have your schedule on all other days/times with every service Except the party slot. This way customers can still book all your other services and the only change would be the staff member selection. Which would be automatically selected.

Hope this helps and isn't that confusing!