Can I use a Grobag Baby Sleep Bag in a Moses basket?

Yes you can, however there will be extra fabric which you can either tuck under the mattress or fold at the end of the basket. If your baby is under 12lb, you could use a Gro-snug 2in1 swaddle and new-born Grobag which will fit into the Moses basket perfectly and can be used with arms in (like a swaddle) or out (like a Grobag Baby Sleep Bag).