Yes and the other particles?? And how could he add the unnatural Lithium... to a pure Ni62 particle or wasn't it that pure at all...
The way he added it was the same way a magician does the long term 'mind reading' trick of what's in a closed envelope. In every performance of the trick, the envelope is opened in the middle time and 'inspected' to make sure there is still a written note in the envelope. That is when the magician performs his trick.
In Rossi's case it could be as simple as opening the reverse side of the dogbone where the planted ash was there all along and no one remembers that the 'actual reactor' was on the opposite side.
I'm not saying Ross is a con artist 100%, but if he is, he is one of the biggest con artists of our time, ever. Truly a great con artist. So great that it begs the question of how the hell did he get caught in Petroldragon?