It can be done ... but there is a world of problems.
The simple answer is to make the radius negative so a 10,000m radius become -10,000m.
Of course your surface layer height limitations will all need to be reversed so high hills will have a larger negative value than low valleys. I *think* the same is true of surface layer slopes, you'll have to check.
Your biggest challenge will be lighting. Disable the sun and place a light source at the centre of the sphere. Sounds fine but you will loose all your directional elements of the light, shadows will only happen under overhangs and trees.
Good luck
Richard
