Hi Christian,
Our MaskedEditBox is designed for validation purpose with certain formats specified with the Mask property and this mask prevents you from entering invalid characters into the control.Currently this control does not support mask for the text entered as you have mentioned.However we will try our very best to include this feature in future.
You can track the progress of implementation through the below given link.
http://www.syncfusion.com/support/features/tools/Default.aspx?ToDo=view&questId=1077
Thanks,
Anu.