You can use the concept of virtual extruders to do this. Essentially, this is the process:
1. Add an extruder in the Extruder tab and name it something like "Infill with different multiplier"
2. Set the Extruder Toolhead Number to be the same as the existing extruder
3. Adjust the Extrusion Multiplier for that.
3. Go to the Infill tab and select the virtual extruder in the Infill Extruder dropdown