Hello Michael Salzlechner,
If by subclass you mean a class that inherits from TextBox, then yes, it is possible to use the spellchecker with that class. All you have to do is implement two interfaces - IControlSpellChecker and IWordInfo which reside in Telerik.Windows.Documents.Proofing namespace (in Telerik.Windows.Documents.Proofing assembly) and load the new IControlSpellChecker with MEF.
More information on spell checking custom controls can be found here.
the Telerik team
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