You can adjust the vertex weights at any time, either by using Weight Paint mode or manually adjusting the weights, weight painting is just a nice gui approach to setting vertex group weight values to vertices.
In your case you most likely want to use a parent constraint to allow you to turn on/off the parenting of the entire object on demand. With rigid objects that don't deform you only need to parent the object to a bone so that it moves with your character.
By keyframing the influence slider you can control when the object is parented to the hand and when it is left behind.
If you are using the game engine then you may want to use a parent actuator to enable/disable the parent relationship. In this case you probably want to set an empty to be a child of the hand bone and then enable parenting to the empty as you can't turn on parenting to an individual bone.