Adding exhaust fan to semi-rigid dryer duct
Bad idea, you should run a separate vent line.

The dryer could overpressurize the duct, preventing your solder fumes from exausting, or could cause gasses from the dryer to flow toward your workbench.  Sure, you could engineer something that works, but the better answer is to isolate the exaust streams.

Dryer ducts can clog, especially at junctions, so you could have it work fine today, but stop working over time /because/ you added turbulence.  Turbulence will increase the amount of dryer dust that is deposited at the site of turbulence.  So joining that line could cause a new problem you don't have now.