Harden doesn't currently have his own signature shoe with Nike, he only has his own version of Nike Hyperchase. It has his own logo on it but it's technically still just a Nike Hyperchase. Signature shoes move the dial with players even with gigantic money numbers attached.
If in fact Harden does go to Adidas, he would join previous Rockets stars Tracy McGrady and Dwight Howard to be the face of Adidas. Howard's contract with Adidas recently wasn't renewed. Howard went to Chinese shoe brand, Peak, shortly after.
Adidas is currently headed up by Derrick Rose, John Wall and Damian Lillard their major brand representatives. Adidas will no longer be the branding for the NBA jersey and apparel, as Nike takes over in 2017-18.
It will be interesting to see if Nike steps in an matches the deal like they did with Kevin Durant this past offseason. Or does that deal prevent Nike from keeping Harden? If Nike does keep Harden, will we see a signature shoe for arguably a top-five player in the league? Will Adidas' poor history with injuries play into Harden's decision?