Welcome to the community, thanks for reaching out.
At this time there is not an available bass module to add onto the Home Speaker 500.
If you would like to increase the bass response of your speaker, I would suggest that you relocate the system. Sound characteristics change depending on location and environment of the sound source. Moving the system closer to a wall or corner, or placing it on or near hard surfaces, will help increase the bass response. You may also be able to adjust bass and treble controls on the connected audio device. However, do note that sound quality may suffer if bass and treble controls are not set properly. Adjust any available tone controls on the connected device to improve the sound quality.