Place the raisins in a small bowl then pour on the rum; cover with plastic wrap and allow the raisins to steep overnight.
The next day remove ice cream from freezer and allow to stand at room temperature until the ice cream is softened but not melting; about 15 minutes. Scoop the ice cream into a large bowl and stir in the rum-soaked raisins, any extra rum in the bowl and nutmeg until the ice cream is well mixed. The rum will soften the ice cream and make mixing easier.
Pack the flavoured ice cream back into the container and refreeze until needed; at least 1/2 hour.