A much simpler explanation is that Rossi added natural copper to the sample. But if you want to go with ANCIENT ALIENS, that's your choice.
The copper and nickel found in the dust would have been "created at the same time ~ in all of their normal isotopes ~ in a super nova explosion. Copper is a tad heavier than Nickel. Assume for a moment that super nova explosions can fuse elements together, or with spare neutrons and protons, if the 5 stable isotopes of nickel present at the creation were subjected to intense heat, pressure and hard radiation (like in a super nova) you would end up with 5 stable isotopes of copper!
So, Voila ~ 5 naturally occurring isotopes of Nickel built up in the fires of creation into 5 naturally occurring isotopes of copper.
A KOINKYDINK obviously.
So, a test for Rossi or anyone else using Nickel. What you do is ask that your flask of nickel filings be used ~ and you've arranged before hand to make sure it's all one isotope.
Then, at the conclusion of the test you check for copper. If it's all one isotope, then we had a sort of "LENR welding" but if it's different isotopes, somebody has some answers to question eh!