How to unpack/repack system.img of android ROM?

I have OnePlus 6T device which has A/B partition system and has a ROM of user type i.e [ro.build.type]: [user]. This device is rooted with Magisk. I have a requirement(Want to place customized sepolicy file under system_root directory ) to modify system.img. I have tried different tools like: 1. simg2img :

OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/img-tools$ ./simg2img system.img sys.raw
Invalid sparse file format at header magi
Failed to read sparse file
OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/img-tools$

2. imgtools

OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/imgtool$ sudo ./imgtool system.img extract
[sudo] password for OMEN: 
system.img is not a recognized image. Sorry
OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/imgtool$

3. and more tools even on windows.. but none of them is capable of parsing my system.img. I have copied system.img directly from OnePlus6T ROM setup which installs Android 9 on this device without any issue. Any help on: 1. How to fix the system.img so it will be extracted and repacked fine? 2. Any command that can unpack/repack system.img? 3. Any working tool to accomplish this task? **Update 1**: I have run file system.img and I found that its ext2 image and the tools support ext4.

system.img: Linux rev 1.0 ext2 filesystem data, UUID=d09c08e9-628d-590e-a610-3a14de2a8db0 (extents) (large files) (huge files)

**Update 2**: Tried to find the magic number and have following result:

OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/imgtool$ xxd system.img | head
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000080: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000090: 0000 0000 0000 0000 0000 0000 0000 0000  ................
OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea/imgtool$

**Update 3** Fing the image already unpacked. So to add the required file, I have mount the image as sudo mount -o loop system.img system_mount and then tried to copy the contents to another folder with cp system_mount/* system/ so that I can add the required file and make new image out of it but I got following errors:

root@OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea# mkdir system && cp system_mount/* system/
cp: omitting directory 'system_mount/acct'
cp: cannot stat 'system_mount/bin': No such file or directory
cp: cannot stat 'system_mount/bt_firmware': No such file or directory
cp: cannot stat 'system_mount/bugreports': No such file or directory
cp: cannot stat 'system_mount/cache': No such file or directory
cp: cannot stat 'system_mount/charger': No such file or directory
cp: cannot stat 'system_mount/charger_log': No such file or directory
cp: omitting directory 'system_mount/config'
cp: omitting directory 'system_mount/d'
cp: omitting directory 'system_mount/data'
cp: omitting directory 'system_mount/dev'
cp: cannot stat 'system_mount/dsp': No such file or directory
cp: cannot stat 'system_mount/etc': No such file or directory
cp: cannot stat 'system_mount/firmware': No such file or directory
cp: omitting directory 'system_mount/lost+found'
cp: omitting directory 'system_mount/mnt'
cp: omitting directory 'system_mount/odm'
cp: omitting directory 'system_mount/oem'
cp: omitting directory 'system_mount/op1'
cp: omitting directory 'system_mount/op2'
cp: cannot stat 'system_mount/persist': No such file or directory
cp: omitting directory 'system_mount/postinstall'
cp: omitting directory 'system_mount/proc'
cp: cannot stat 'system_mount/product': No such file or directory
cp: omitting directory 'system_mount/res'
cp: omitting directory 'system_mount/sbin'
cp: cannot stat 'system_mount/sdcard': No such file or directory
cp: omitting directory 'system_mount/storage'
cp: omitting directory 'system_mount/sys'
cp: omitting directory 'system_mount/system'
cp: omitting directory 'system_mount/vendor'
root@OMEN-by-HP-Laptop-15-dc0xxx:~/WorkArea#