Hi do you think it is also possible to run an El Capitan in virtual machine to download Sierra, of course from unsupported mac?
Yes. I said before this can be done with a Sierra or El Capitan virtual machine. If you want to do it via El Capitan, make an El Capitan virtual machine. How to do this will differ depending on which virtualisation software you use. Whilst in the virtual machine go to mac app store, sign in.
Then there are 3 ways to download the Sierra full installer inside the virtual machine.
1. If you have previously redeemed a code for Sierra go to purchases tab and click Download. Ensure first there are no older Sierra installers in the virtual machine.
2. If you are a public beta tester, go to https://beta.apple.com/sp/betaprogram/ log in and redeem a code for Sierra.
3. If you are a developer, go to https://developer.apple.com/macos/ , click Download, login with developer account, find the GM and click Download again and a new redemption code will be generated and be redeemed in the mac app store.
Doing this inside the virtual machine means it won't matter if you are running it on a machine which is not directly supported.