Hi rafael, welcome to the forum. Congratulations on using KARMA scenes to make variations. To create custom drum modules with KARMA, you have limited options using the Kronos alone. Purchasing the KARMA software will give you access to more KARMA parameters, and let you directly create patterns and save them on to your Kronos to use independently from the computer.
Here's a link to the KARMA forums, where you can find detailed information.
Be aware that KARMA may or may not be a suitable or worthwhile solution for custom drum fills.
Since you dived into the deep end successfully using KARMA for variations, and yet you are new to your Kronos, I am not sure how much of a foundation you have with the Kronos architecture, specifically regarding drums and drumkits and patterns. I was going to start this post by laying the foundations first, but quickly realized I would be writing a book. So here are some tips from the opposite end of the spectrum -- I will give you high-level suggestions, and leave it up to you to ask for clarifications if needed.
1. How to make better drum fills (outside of KARMA)? From the Drum Track page found in program and combi and sequencer mode:
The Preset drum patterns are grouped by groove, and you can change the pattern while the drums are playing. If you change in the middle of the 8 or 16 bar phrase it is currently playing, it will instantly play the new pattern from the middle of the phrase. So, if you switch back to the old pattern before the new pattern reaches bar 1, you can "steal" that part of the new pattern to use as a drum fill.
Use Shift to move the whole kit over by a few MIDI notes. The MIDI pattern will continue playing, but instead it will trigger different notes. This can be used as an alternate groove entirely. Or, like above, you can Shift back to revert to the normal drum kit, and just steal a few bars to use as a fill.
For both of these methods you will need to do it live. There is no MIDI cc for these. (But if you're playing in sequencer mode, you can record your manual changing of the shift or drum pattern. This will be recorded as sysex, which you can use in an RPPR pattern. Closest thing Kronos has to macros).
Ok, this post is already too long. Here are my other tips in a few words.
Go to sequencer mode, create some User patterns, convert these to Drum Track Patterns. Go back to the Drum Track page I mentioned above. Instead of Preset Drum patterns, select your User Drum patterns. see Julius DeBerry youtube tutorials on Drum Tracks. Also Jurgen Kohler. And Stephen Kary / Karma-labs video on KARMA drum fills.
If you're already performing live in sequencer mode, use RPPR for custom made drum fills. Either actual drum fills, or as mentioned above, as a macro.
Split your drum kit over a couple timbres. Mute the main one occasionally when you want to do a fill. Enjoy the silence, or manually play a fill before switching the main drum groove back on.
Sample a drum fill from outside the Kronos. Make that into a program (in Sampler mode, convert MS to a program).
Or, play a drum fill yourself, or via KARMA, and record live (sample) what you are doing (far right of Kronos -- Sampling Rec and start/stop buttons).
From within KARMA on the Kronos (less precise results, more random):
Go to GE RTP Scenes page. There are total of 32 parameters you can mess with (top buttons 1-16 and 17-32). Experiment with KARMA parameters beginning with Drum, Rhythm, Index, and Note series while the groove is playing. You can Assign ones you like to KARMA sliders and switches (maybe after first un-assigning a paramater you don't need).
Ok, that's it. Time for a caffeine refill. Good luck!