Could be done. Major PITA, lots of dicking around. Not like to accomplish much useful.
Oh yeah. A custom mold. How much are you willing to spend?
Get a mold, accurately sized to cast a half of the ball you want to end up with, think like a empty cantelope skin. Or a hard boiled egg half, with the yolk removed.
Cast it in the alloy of your choice.
Carefully hold in alignment while applying a solder of a lower melting point than the alloy between the two halves.
Install into temperature controlled oven and slowly, oh so slowly, raise the temp up until the solder 'just' flows out and fuses the two halves.
So yeah, it could be done.
Why?