Please take a look at my suggest in this question:
https://www.telerik.com/forums/radgridview---grouping-and-filtering-for-n3-numbers
I think that you can offer a great workaround by exposing some functionality.
Currently, the column filtering list is created over the raw values to which the column is bound to and eventually, if an IValueConverter is applied to the DataMemberBinding, just the visual appearance of the value is changed. Grouping, on the other hand, works differently. If there is a converter applied to the DataMemberBinding of the Column and you group the Grid on that column, the groups will be evaluated over the converted values.