SPRadGrid supports the native SharePoint field types, however, the Q3 release version is the first to sport the possibility of binding the grid to custom field types. As a consequence this will give you the chance to binding the grid to your own content types. The only requirement to keep when developing the custom field type is to specify a display mode template and not just a default one.
In the example below, SongName is a custom field type which is connected to a Song list. The field rendering control features a RadComboBox in edit mode that retrieves the names of all songs in the Song list.