Gary
The Casio rhythm files are MIDI files, but they are Casio proprietary MIDI files. You can't just take a MIDI phrase and give it an .AC7 file extension and get it to work. There is other required data. I do not know what Casio calls this "other" data, but Yamaha calls it "the CASM". Among other things, it tells the keyboard how to handle different types of chords - majors, minors, 7th's, and so on for that particular rhythm and is dependent on the specific melodic passages for the various parts. Without out that, the rhythm will not play correctly in all situations. Writing CASM type data is a programming language all its own, and there are several folks on the Yamaha arranger forum who have spent a good portion of their adult lives reverse engineering Yamaha styles and learning it.
Anyway, there is an old converter program on the web that will convert MIDI files to the old .CKF Casio rhythm file format, and then the keyboard itself will convert the .CKF format to .AC7 when it loads it, but it is really tricky. There are several guys over on the Casio Music Forum that have tried it. The tedious part is getting the various sections, intro, main, fill, ending, properly marked in the DAW. If a section tag is off by as much as one clock tick it screws up the entire file, and deciding where the tags go is kind of like shooting in the dark. Of the several who have tried it, a couple gave up in utter frustration, and the third was successful, but said he would never try it again. For the amount of time and effort required, the failure rate is entirely too high. About a year ago, I spent 3 weeks using a software program written specifically for trimming sections from large Yamaha style files to run on Yamaha boards that require smaller files (one intro and ending instead of 3, two mains instead of 4, etc), and I was never able to get that working. CASM type data is non-MIDI data, so the typical DAW or editing program throws it away. Then, when you save your efforts, there is no longer any CASM type data, so the file won't work.
So, back to your original question, Yea it's possible with enough blood. sweat, and tears.