How to sew in a single page into a book
If you are looking to add signatures to a completed sewn binding, you would have to unpick the binding, add the signatures, then re-sew the binding.

Individual pages (leaves) cannot be sewn into the signature unless they have an excess at the binding edge which you can fold into the signature crease.  Without the excess, you will need to glue the leaf to their neighbouring leaves with a thin bead of glue as close to the binding as possible.  This will be the most secure possible.