Yes it is possible only on summer Saturdays and it means a late arrival into Margate:
17:03 Depart Scarborough Arrive St Pancras 21:06 (Using East Midlands Trains)
21:42 Depart St Pancras Arrive Margate 23:10 (Using Southeastern via HS1)
The Super Off-Peak Single for that will cost £119.90, this is valid at anytime on Saturday and by any train (though only the above will go direct to St Pancras via Sheffield). I'm not sure about splits to bring this down but I'm sure someone else will be able to advise.