Unless there is some sort of trick with using the bootloader from another UEFI compliant OS I doubt this will be easy to do without invoking the CSM. You may also run into issues if the HDD/SSD is formatted as GPT (GUID partition table) rather than MBR (master boot record).
If you specified what OS are you are trying to boot into you will likely get better help than a generic question.
Personally, I would just use a virtual machine to run the legacy OS inside a UEFI compliant OS. If you have thunderbolt and a thunderbolt video card (say from a dock) you can have native full speed graphics support inside the virtual machine. Though it is quite complicated to get working.
On a side note, I remember that Windows 7 was unable to run on my Surface Pro 3 due to it not supporting the newer UEFI standards that dealt with the video bios during booting. I can't remember what exactly would be required to make it work, but it was certainly beyond me at the time and no ready made solutions existed. If you are attempting something similar, you may as well give up and use Linux or Windows 8.1/10/11.