I think you risk the inner duct becoming a condensation surface and potential surface for mold growth. Also the connections for these two systems are fundamentally different. The forced air system is for space cooling or heating (recirculating and filtering inside air). The HRV or ERV is for exchanging exterior fresh air with interior stale air. The most efficient paths for each system are different. The difficulty in sealing the HRV ducting entering and leaving the forced air duct is probably not worth the effort.

Torben is right. The forced air system is for space cooling or heating (recirculating and filtering air). The HRV is for exchanging exterior fresh air with interior stale air.
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broan parts