Hi Yoni,
Thank you for using Syncfusion product.
You need to implement a custom sorting to do this. You can add a custom IComparer to the SortColumnDescriptor for this column. You do this by setting the Comparer property on the SortColumnDescriptor. In your custom IComparer object, you can sort based on any criteria of your choice.
When you do custom sorting in the grid, then you need to use custom grouping for handling the groups.
Please refer these forum threads
http://www.syncfusion.com/support/Forums/message.aspx?MessageID=46737 See the following KB article for more details on this.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=73Please let me know if you need any further assistance.
Regards,
Suba