Failed to packing mod

I want to pack .uasset file to creat a mod for Romancing SaGa 2: Revenge of the Seven. This game was built by 4.27.2 with IOstore, so there are .pak, .ucas and .utoc. The main game data are in .ucas.

My packing steps:

  1. I installed offical Unreal Engine 4.27.2, and built new project.
  2. I modified DT_AllFlagSetupTable.uasset which is under Content\Main\Flag\Data. This file was exported by Fmodel. It is a game data table file without any models and textures.
  3. Modified DT_AllFlagSetupTable.uasset by HxD. This is game flag data, I only changed 1 flag value.
  4. I copied Romancing SaGa 2’s main folder to UE4 project/content.
  5. Edit Top → Editor Preferences → Experimental Phase → Allow chunk ID assignment
  6. Edit top → Project Preferences → Project → Packaging → Use pak files/Use IoStore/Generate chunks
  7. Packing mod. it failed with report “Error: Package …/…/…/…/RSAGA2/Project/Content/Main/Flag/Data/DT_AllFlagSetupTable.uasset has malformed tag”

I think the main issue is the UE4 new project doesn’t recognize the .uasset file from other game. How can I import .uasset file to UE4 new project? Thanks!
Or what’s tool can pak UE4.27.2 mod to .pak, .ucas and .utoc? I tried to use IOstorePak, it created these three files, but there were no content in these files.