can I put monetized stuff in the game
Yes you can.
How you would do it is a bit more complicated. Typically, .io games will use some sort of advertising service. Search the forums here for @devMidgard posts as he talks a bit the services he used for his games.
If you are talking about paid items, you would have to provide some for sort of user account service (be it your own, Facebook, Twitter etc) so you can keep track of what items the user has brought.