The real situation is a "black box" current source capable of supplying 0.5 A DC current when connected to a 1K Ohm external power resistance.

Connecting two black boxes in parallel results in a doubling of the current value.

The open-circuit DC voltage is practically zero.

What is the best principal/practical way to convert this current source into a practical voltage supply ( high voltage DC or rather ultimately 110/220V AC ) without any modification of the black box?

What is then the best possible conversion efficiency?