You can probably do it the reverse way.
Starting from Win7pese to add then win10pe wims files.
Or you should be able to do it manually by copying yours patched win7 (6.1.7601) BOOTMGR1,2,.. in your Win10PESE build
and also BC1,2 (copied from win7 or maybe just win10 BCD renamed).
It should allow to mix between win10 EFI and windows 7 bootmgr
To test, Let us know