Yes - you can use bath additives.
However, I am not completely satisfied with the product. The bubble function is very limited, I miss a direct massage function.
Website Beurer:
Do not use essential oils to avoid damaging the footbath. Only water-soluble bath additives and herbs may be used for all foot baths: Warm foot baths with arnica, rosemary or sea salt extracts get the circulation going. Lukewarm foot baths with lavender or thyme are a blessing for stressed feet.
The water is not heated, so to speak. It should already come out of the tap at the right temperature. In any case, the product is not recommended for warming the feet ;)