single light fixture on my ceiling fan

Only if the wire gauge is sufficient to carry the additional amps.

Yes, you can replace a single light fixture on your Harbor Breeze ceiling fan with a compatible multi-light fixture by turning off the power, removing the old fixture, and connecting the wiring following colour codes (black to black, white to white, green/bare copper to ground), mounting the new fixture securely, and testing the lights before restoring power. Ensure compatibility and consult the manufacturer's instructions for your specific fan and light kit, or consider seeking professional assistance for a safe and proper installation.