My mixing desk only supports stereo outputs. To connect a mono grotbox i need a mono signal. Is there an Adapter or cable that feeds the two stereo signals in one mono Jack or xlr?
Simple answer is if you are feeding a stereo signal to a single input you need the signal to be summed so you do not lose any elements.
To do this you need 3 resistors in the XLR end, diagrams are available online however its not a simple solder job. If you shop around you can get stereo to mono cables but make sure they have resistors and are summed to one pin. The 3rd pin in the XlR mono should not be used. Otherwise you'll get a weird effect.
You don't say what size you require, but looking at somewhere like StudioSpares you'll see what is available. Failing that, buy the jacks you want, get a soldering iron and ideally a test meter and then you can make your own leads. There are plenty of video tutorials online to guide you through……it's a good skill to have and only takes a bit of practice.