There are a couple of ways you can go about this.
RF is designed to apply dual rates and expo using settings that you'll find in the Aircraft Editor. RF uses what's called the "software radio" to virtualize the radio settings that you'd find on programmable radios, giving you the ability to customize those settings when you're using one of the InterLink controllers.
If you'd like to change the rates and expo in the Aircraft Editor, I can point you to a writeup I did
here about the InterLink DX. The section about rates and expo starts on page 7.
The other way you can do this is on your iX20, as you suggest. To get the iX20 working with the computer, you need a USB receiver dongle such as the Spektrum WS2000. One drawback of the USB receiver is that it's limited to eight channels, so that's all you'll be able to use even though you have a 20-channel radio.
When you set up the WS2000 in RealFlight, you'll use the Controller Profile to map channels from your iX20 to the virtual channels in RealFlight. As part of that Controller Profile, you'll find a check box for selecting whether RF should use the software radio to control rate and expo. If you turn off that box, then RealFlight will keep the rate at 100% and expo at 0% all the time, and then you can control that with your iX20 instead. I have another writeup
here all about the WS1000/2000 USB receivers. I used a DX9 as the transmitter, but I'm sure you'll be able to apply it to the iX20 with no problem.