No, do *not* solder a capacitor onto the bottom side. It will cause problems!
While Andrew did put some code into the USB Host Shield library, but reality is USB host isn't really supported on Teensy 3.2. To use Andrew's code, you need to seriously hack the hardware so another board can program it.
Today, the various USB host shields are the only (just barely) workable solution for USB host.
Long term, Teensy 3.6 will provide USB host. Today there's no working code, but I intend to change that situation! The 3.6 board was designed with the necessary capacitor, and also with a software-controlled current limiting switch to control it. The controller is a full EHCI implementation, which for now is difficult because it requires a ton of software work, but long term it will be excellent when the software matures.