Hello Michael,

The Spreadsheet does not allow customization of the Filter menu out-of-the-box. There is an existing feature request in our Feedback Portal about it, so I suggest casting your vote for it.

A possible workaround is to bind to the click event of the .k-icon element, and prepend the button to the .k-action-buttons div.

$(".k-icon").click(function(){
  $(".k-action-buttons").prepend("<button class='k-button customBtn'>Custom Btn</button>");
  $(".customBtn").click(function(){
  	alert("Your custom code")
  })	
});

Here you will find a small Dojo example for reference.

Feel free to ask if you have any questions.

Regards,
Martin
Progress Telerik