RadGrid does not support mixing declarative grid columns with grid columns added dynamically at runtime. You should either
create all the columns in the grid programmatically, or else define them all in the ASPX file. A suggestion would be to use a header context menu to add columns dynamically to the grid. Check out the demo below to understand where they use the headercontextmenu to show or hide columns: Header context menu
For additional information, you can also refer the link below: Programmatic creation