Nickel alloys (also found in thermocouple wire and battery terminals) soft-solder well, but the flux should be the acid type; anything recommended for stainless steel will work fine.
The common type includes some sulfuric and hydrochloric acid; keep it well away from anything that can rust (there's some fumes emitted). It doesn't take much flux, just a drop on a toothpick. The toothpick will blacken and turn to goo after a minute, though.