One way I can think it could be done would be to use an afghan hook for the crocheting part. Afghan hooks have a stop on one end (like a straight knitting needle), and a crochet hook on the other. I think I've also seen some that are straight with crochet hooks on both ends. But I can see a scenario where when you were knitting and got to a part where you wanted to switch to crochet, you could load the stitches onto the afghan crochet hook and then create a pattern utilizing that. It makes a unique, tapestry type appearance. The patterns I've seen with the afghan hook you go across one row with all the stitches on the hook, then the next row you come back across and end up with one stitch on the crochet hook, then the next row they all get loaded back on the hook again, etc. If you ended a pattern where all the stitches were back on the afghan hook again, then you could pass them back across to a knitting needle and go back to knitting again.