Some arrangers, including the E-A7, Genos, etc., have assignable switches that might also be assigned to trigger fills.

If your arranger doesn't have this, there might be another option... A company called MIDI Solutions makes an electronic module called a Footswitch controller. The module has a 1/4" jack and a MIDI din jack.

A footswitch is nothing more than a momentary, normally open switch. You can buy push-buttons just like this from online electronics suppliers.

With a little soldering, you would wire the push-buttons to the 1/4" jacks on the MIDI Footswitch controllers. Now comes the brilliant part... At least Roland and Korg have MIDI implementations that are complete enough to trigger any of the fills with MIDI messages. So you push a button, it completes the circuit, and the MIDI Solutions module tells your arranger to play a fill. Using this method you can add "fill buttons" to an arranger that doesn't have them.

The only remaining problem is the ergonomic considerations of mounting the push-buttons. The front fascia below the keys puts them in reach of your thumb (i think.)

Sometimes as an end user we have to get a little imaginative to make these marvelous musical tools do our bidding!