I don't think you can change the range of an FX envelope, I've run into the same situation and just thought of a possible workaround,...
1) Create a JSFX with slider in the range you want, (I'm only testing 0 - 20).
2) Add the JSFX to the track and make an envelope for the slider, set the envelope to its max value.
3) Open ReaControlMidi, select the param you want to control and link it to the JSFX slider, adjust the offset and scale as needed.
4) Now use the JSFX slider envelope to adjust the ReaControlMidi param.
"Range" is bad name for slider, should be named "value" or something, my first JSFX.