There isn't a simple way, but there is a trick that works. Centre the object to the workspace using Edit>Locate>Center About Origin. In any non-perspective or otho view (eg. front or left) create a small temporary object, such as a sphere or cube primitive, beside the object and mirror it to the other side, select all 3 objects and Build>Join Solids. The new object's origin point will default to the centre. In vertex editing mode, delete the points of the two temporary objects and voila, you are left with your original object with mathematically centred origin. Points to consider are to ensure the temporary object is smaller than the main object and is centred on 2 of the 3 axes so as to not cause the reset origin to be off centre, and to ensure that the temporary object is clear of the main object so it, and it's mirror, can be easily deleted in vertex mode.