robertferanec , 06-06-2020, 03:17 AM
If this symbol is from the Altium's IEEE symbols (Place -> IEEE Symbols), I think, it can not be modified. I heard, that some people use it as a template, they re-draw it (they draw on top of that by using wires and polygons) and then just delete it.
If you draw the symbol by yourself, and you would like to fill it up, I would use combination of filled ellipse and filled rectangle.