Hello baroa1989,
Yes, there are already stevia liquid products. Just do a little research on the Internet. In addition to online stores that offer such liquid products, you will also find information pages that explain the dosage, application, etc. An example would be http://www.steviafluessig.de/stevia-fluessig-dosierung.
Best regards

New at Coop: Assugrin Stevia Sweet Liquid
https://www.coopathome.ch/de/lebensmittel/grundnahrungsmittel/mehl-%26-zucker/s%c3%bcssstoffe/assugrin-stevia-sweet-fl%c3%bcssig/p/5696794