Here's what I did:
What you need:
Windows 7 SP1 x64.iso
Windows 10.iso
Microsoft hotfix
https://support.microsoft.com/en-us/kb/2990941
NTlite https://www.ntlite.com/download/
First request and download the hotfix, e-mail arrives instantly with a link.
1. Download and install NTlite (or other similar you prefer)
2. Extract the Windows 7.iso to a folder.
If you are installing in UEFI mode, check efi/boot there should be a bootx64.efi. If not grab it from a current installation of Windows 7 x64. C:\Windows\Boot\EFI\bootmgfw.efi copy/paste and rename it to bootx64.efi.
3. Using WinRAR or 7-zip extract the Microsoft hotfix.exe file.
4. Run NTlite and open the iso folder.
You will see 2 folders your current windows and 2nd the iso folder open and choose the version you want to install, home, ultimate etc. and press load, or right click load. It takes sometime..
4.1 Go to updates and add package file. Add the extracted hotfix Windows6.1-KB2990941-v3-x64.msu
(for experienced users: you can add more updates if you want to avoid downloading the huge list of updates everytime you install windows 7 important to keep install.wim under 4GB due to UEFI/fat32 limitation)
4.2 Go to Apply press Start and wait to finnish.
When it's done close NTlite and go to iso folder and open sources folder.
Delete boot.wim and setup.exe
5. Open Windows 10.iso with winrar and extract the boot.wim and setup.exe to your sources folder of Windows 7.
6. Copy all contents of iso folder of Windows 7 on a usb drive.