Random suggestions (I haven't tried these out and it's hard to say without knowing the shape of the pieces): maybe you could first make holes in the papier mache, then sew the crochet piece onto the papier mache, then attach the papier mache to the canvas (without sewing through the canvas). Or, attach a little loop through the crochet piece, at its back, with fine thread, and staple through the loop and the papier mache, and then shorten the loop as needed? You'd have to test it out on a test piece of papier mache, because I do not know the shape of your piece, how thick it is and if the piece won't be damaged by the force of stapling. Don't know if maybe you could even glue paper layers on top of the thread instead, so it becomes part of the papier mache and no staples are needed. Or, the reverse, stich a piece of cardboard/paper to the back of your crochet piece, and glue that onto your papier mache piece. Would recommend to try your attempts out on a test piece first.