If I understood correctly, then here is what I would do:
1. Take the IFC PLN file. This would be PLN-1.
2. Save the TW file as PLN. This would be PLN-2.
3. Match the Story Structure of PLN-1 to PLN-2.
4. In 3D, fix the vertical "offsets" of elements in PLN-1 that possibly resulted from changing Story Heights.
5. Merge PLN-1 into PLN-2.
6. Share PLN-2 as a Teamwork file.
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