To enable NVMe support to Windows 7 you need to add KB2990941 and KB3087873
The issue is that this Hotfix is not available from Microsoft anymore, which explains why it is not automatically added to OSBuilder. The only place I could find the MSU file was on a Third Party website.
To add these Hotfixes to OSBuilder simply download the attached XML file and save it in C:\OSBuilder\Content\Updates\Windows 7 x64 7601
For Windows 7 SP1 x86, save this file in C:\OSBuilder\Content\Updates\Windows 7 x86 7601
Simply run another
Update-OSMedia -DownloadUpdates to get the added Hotfixes and you should be ready to go
For Windows 7, only the Servicing and Cumulative Updates are applied to WinPE. For the NVMe Hotfixes, I change the Severity of the update to WinPE. OSBuilder will automatically import these Microsoft Updates in WinPE for Windows 7 as long as this value is present.