Are we capable to write custom partition (like TWRP) in HUAWEI phones with BL Locked ?

Yes but you will need Resurrector module activated !

Rebrand huawei phones:

1. Get oeminfo from needed customization.
2. Make custom.txt with string "OEMINFO=youroem.img" OEMINFO.img must be 64Mb.
3. Write with advanced option HuaweiRessurector and unticked "Skip write oeminfo".
4. Reboot.
5. Write full FW from FG support.
6. Write update customization from FG support.
7. Profit.

Be careful, some Huawei models (FRD-AL00 for ex.) dont want to have NON-China region because of huge amount of bugs.
I've done at least STF-AL00 to STF-L09 and PRA-AL00 to PRA-LA1 conversion.
There are no "Null Imei" and other network problems.