These days I am going through the source code defined in FbxImported.cpp, AnimSequence.cpp and SkeletalMeshEdit.cpp and find that we can indeed technically access the controller and data model by which raw animation curves can be readily manipulated. I am thinking if i can extract the data and display them in the sequencer editor and provide modifiability. Another workaround, the Control Rig, seems to serve the same purpose but it looks quite complicated and is not as easy-to-use as i thought.