Customizing Set Grid Row Bitmap System Function

Does anyone know how to customize the Set Grid Row Bitmap System Function? Currently, it allows you to pick from a list of 15 options. (I.E. <Check Mark>, <X Mark>, <Action Msg>, <Sigma>, etc.) and displays the applicable bitmap. I would like to add my own custom option, say <smiley face>, and attach it to the grid instead of a <sigma>.

Thanks,
Kip