The ASP.NET AJAX version of the RadSpell control is almost identical to the classic control, and the properties of both controls are also pretty much the same. I checked the RadSpell for ASP.NET server API but did not find a separate property for the custom dictionary path. Custom dictionaries are stored in the same folder as the normal spell dictionaries. There is a property you can use to control the name of the custom dictionary (CustomDictionarySuffix), but it is only applied at the end of the file name and cannot change the file path.
If you do not want the custom dictionaries to be in the same folder as the .tdf files, you need to write a class that implements the ICustomDictionarySource
interface like Rumen showed in his message. The implementation is fairly simple - you open the physical file in the class constructor, read a line from the file each time in the ReadWord
() function and seek to the end/write a line in the AddWord
the Telerik team