I installed my Keystoker Koker with the configuration you are talking about. I divided the Koker's output to equally feed my two zones (upstairs and downstairs); and, tapped into the cold air return to feed the Koker's convection fan input. This configuration worked OK; but, the Koker's 1530 CFM fan just didn't provide enough flow. The Koker had plenty of BTUs available, I just wasn't moving heat around the house as well as I wanted. We didn't have many sub-zero days last winter; but, when it was sub-zero, I couldn't get my upstairs above 68*. During most "normal" winter weather, the heat distribution throughout my house was outstanding---the whole house (2 story 4000 sq ft colonial) was +/- 1 to 2 degrees.
I'm going to convert my system to the LL suggested configuration during the off season: my Koker's output will feed my existing FHA furnace cold air return input. I've already completed the wiring. The FHA furnace convection fan (~3000 CFM) wlll be enabled by the Koker's convection fan thermostat.
If you only have one zone; and, you're confident the Hyfire II's optional convection fan (?? CFM) will move enough air for your application, piping the furnace output directly to your supply duct work is a viable solution.