"With regards to actual "training" either in the sim or aircraft... A rated ATP can do it under the provisions of his licence as long as it is in accordance with your ops manual."
Only in the US and only under US Part 135 or 121. Furthermore, it can only be done after receiving FAA approval of your company training program.
An FAA ATP cannot just give instruction whenever he feels like it, and certainly cannot do it outside of the USA, even on N reg to FAA certificated pilots. Be careful of the legalities and study your FAR's before doing anything like this.