Rolling out BitLocker in a heterogeneous environment without System Center Configuration Manager (SCCM). How to create a new boot partition for a laptop where Boot and Windows are located on separate disks

BitLocker can’t be enabled, because the hard disks are using the MBR partitioning scheme.
The partioning scheme can’t be changed from MBR to GPT because Microsoft’s tool mbr2gpt says the disk layout is invalid.
The disk layout is difficult to fix, because the boot partition and the Windows partition sit on separate disks