GridMarket: Can you make a child theme of a child theme?

This may be a stupid question: I would like to make modifications to GridMarket, which is a child theme of FrameMarket. Can I make a child theme (in the usual way) based on GridMarket? Or do I need to make a complete copy GridMarket (which points to FrameMarket, not the GridMarket child theme) and modify that? This later option still makes it difficult to easily incorporate GridMarket updates -- unless I'm missing something.