Unfortunately, without adding an audio processing unit/amplifier (receiver) your options are very limited. If you're absolutely set on not adding another component, your option is to find a pair of self powered speakers.
For example,
http/www.ebay.com/itm/TECHNICAL-PRO-SELF-POWERED-MINI-SPEAKERS-/221276895242
The downside is they will eat up another set of audio out on your TV (if you have any left). However, if you're using the optical connection for your soundbar, you should have an available RCA out connection on your TV to go into these speakers.
One word of caution I would have is the possibility of echoing with this type of setup. The optical may transfer the audio at a slightly higher rate to your soundbar versus the RCA to your speakers. Meaning, the audio will be delayed for a split second between sets of speakers giving off an echo affect. Sometimes you can duplicate this between your TV speakers and your soundbar. Of course, this may not happen at all, it depends on your setup.