There are already lots of CAS files archived just search for MSX TOSEC however if you open the CASToolsGUI and drag a WAV file in to it you will be able to try to convert it to CAS. You may have to try several different options before you get a successful CAS file.
I have also been converting some MSXDev entries to CAS as well as some of my own games that haven't been archived and they can be found either on the Arduitape Facebook page or my Arduitape blog.
If you ever need help just PM me and I'll do what I can.