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And the third, complex way:
1. export pants and body to blender.
2. modify the pants mesh by moving the vertexes. don't delete or add or join any vertexes.
3. follow the guides on how to import the modified mesh as a morph
4. repeat blender adjustments until you get what you want
Fourth way:
Find an asset that has morphs for it, and import the morphs into your asset.
The success is not guaranty, but auto-fit increase the global compatibility even if the meshes don't have the same geometry.