I'm no expert in these matters, and have only recently been discovering information about all the different file-types and possibilities with Casio's ... But I found they offer their IDES v4.0 software for the WK-3800.
http://www.casio-europe.com/euro/emi/cms/ This has a rhythm editor/converter built into it. If I had to tackle a project like that I would start by recording the CTK-611's rhythm into any good MIDI recording software. (On my CTK-5000, there's a MIDI option to set for "Accompaniment Out", making this easy. I don't know if the CTK-611 does this.) Record all the various parts (intro, normal, variations, lead-out). Clean them up for one loop of each, and merge them in the required order. Then load that into their rhythm editor, setting the proper break-points, banks, and tones (see their appended info files on tone parameters, banks voice-numbers, etc.). It's how I'd start. But before that, I'd go through the 503(+?) different downloadable rhythm files I found on Casio's web-sites (see my post
https://www.keyboardforums.com/163-free-extra-rhythms-casio-ctk-5000-a-t22540.html ) to see if those rhythm's or ones very similar aren't already available without the steep learning curve of using their rhythm editor.
Another simpler option: let the 611 play the rhythm while you use the 3800. Stack 'em and look like a "pro".