Hi Nigel,
There is a voice called 'Rubber Bass' already on Genos, but it is hidden away in the 'Bass' category of the 'GM&XG' folder which can't be directly accessed from the panel, except in MIDI Song mode (thanks Yamaha!).
However if you check out the post at this link:
https://www.psrtutorial.com/forum/index.php/topic,44058.msg347642.html#msg347642 Stijn tells you how you can defeat Yamaha's petty restrictions and regain access to these voices, allowing you to use them in keyboard parts.
Unfortunately the link to download the voices in his post is no longer active, but if you scroll down the thread to Eileen L's reply (#2), the attachment in her reply can still be downloaded. The attachment contains the voice data files (.vce) and is not very big. If you unzip the download to a USB stick and connect it to the keyboard, you can check out the voices directly from there and if you prefer, install any voices of interest in your USER drive later. You don't need to use any expansion memory or YEM for this.
Regards
Ian