Retargeting non-hierarchy spine.

I have a Skeletal Mesh with that hierarchy
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And i want to make IK_Rig and Retargeter for that target skeletal
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The chest retargeting is not working correctly because the chest joint of the target skeleton is outside the hierarchy. Is there a way to perform retargeting without modifying the target skeleton?
I created a separate chain in the source skeleton from spine01 to spine04 and for the target skeleton from spine01 to spine02. I also created a separate chain for spine05 in the source skeleton and spine03 in the target skeleton, but retargeting still occurs with bugs.