[Hugh]

"I'm wondering how to graft a piece of wood in to repair it...

Sand/rout it flat and glue a piece on with as little glueline as possible showing??? Shape the new piece to fit the existing contour and let the glueline be what it wants to be??? Dovetail a new piece in???

What kind of adhesive??? Glue??? Epoxy??? Polyester resin???"

Postino,
This may be too late, you may have already repaired your grip!

Sand the grip area to be sure it is completely flat.
Sand the repair piece to be sure it is completely flat.
Be sure they will mate up with minimum to no space between the two pieces.
Be sure the grain of the wood in the two pieces runs in the same direction, and the closer they are to the same color the better.

Apply super glue to to grip, stick the repair piece on, move it around a little to spread the glue evenly onto both pieces.
Hold them together for a minute or two until the super glue dries. Wait an hour or so before sanding to shape, to allow the glue to cure completely.