Hello Ales,
This functionality is not supported in the Kendo UI NumericTextBox, and unfortunately it does not support custom text masks. A widget called Mask Input, where custom masks could be added, will be released in the Q1 2013 release. The beta for it is scheduled for mid-February. You can check this in our Road map section:
http://www.kendoui.com/web/roadmap.aspx
Regards,
Kiril Nikolov
Telerik
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