I don't think you can project normal map details back onto a mesh. You could try converting it to a height map and doing it that way, with Max or Maya.
I'd probably just do it in substance painter. You can paint details right onto the normal map.
What are you trying to do? Add more detail? Increase the strength of the normals? Re-engineer a mesh you don't have the high poly for?
Why are people assuming he is re-selling the model? You hardly buy stuff that you don't need to modify to suit or project/game/whatever.
To the point best case scenario would be having the high poly model but since you don't, using something like ndo to add normal map detailing might be an option.
I suppose it depends on your license. You need to check to see if you're allowed to modify it, and in what kind of settings. Sometimes modifying for commercial use is against the license policy. I didn't think it was reselling directly but rather using it in a game or for a portfolio piece.
Either way, if you want to just add some detail to the normal map then check out substance or quixel.
I'm not reselling the model, i'm using it in my own game. The license clearly states that i'm able to modify, otherwise, i wouldn't have bought it. What's the purpose of buying something that is royalty free and not being able modify it? All games would look terrible. :P
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I'd probably just do it in substance painter. You can paint details right onto the normal map.
What are you trying to do? Add more detail? Increase the strength of the normals? Re-engineer a mesh you don't have the high poly for?
Precisely.
To the point best case scenario would be having the high poly model but since you don't, using something like ndo to add normal map detailing might be an option.
Either way, if you want to just add some detail to the normal map then check out substance or quixel.
https://www.allegorithmic.com/products/substance-painter
http://quixel.se/