I can't recall any transit desk where you can check a bag. I suspect you will have to clear customs and get it checked in at the regular check-in desks.

There are also "Gate Check" options, where a gate agent decides your bag needs to be checked luggage. Not sure if you can ask them or not...

Could you explain why you think you need to get a bag checked in? Liquids? Perhaps better advice can be given if you let people know why.