The NumericTextBox widget is designed to allow entering of digits, negative sign and decimal separator.
Currently, there is no built-in logic, which will allow you to implement the required functionality without modifying the source code.
As a side note, the users which use GPL version of Kendo UI need to use public forums. You can find more information here.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI