Is it possible to pipe variables into the rows of a Checkbox Grid from a previous Radio Button Grid question?
The system won’t display the Radio Button Grid as a valid choice.
If you have a PRO or ENTERPRISE level account you might be able to develop a custom script. There are working examples of custom scripts available for review in the Developer section. This page lists a couple of scripts that pipe responses from Radio Button Grids to other question types. There is not a script that matches your requirement, but you may be able to modify one of the script to meet your needs.