Hello, is it possible to add multiples keywords for my “mood” category ? I want to add for instance Stressful, Dissonance.
In general yes, although it’s unclear exactly what you are doing. I assume you created a User Attribute named “Mood” which is of type “Text”.
If you double-click on “Stressful” it should highlight the word and allow you to type in different words. The dropdown list of words shown when you just select the field are all the various text values which are currently in use by the Mood field. You can’t create the text tags in advance, just do it as needed and they will be added to the list.
I’ve noticed that in the Media Bay sometimes it is a bit tricky to hit the exact right spot to double-click.
But when I do that, the new word replace “stressful”, I want to add it alongside
This is not possible, if you want to attach multiple tags in one section (tab) you are limited to “character” tab where you can tick various options (analog, dark, fast attack, one shot, loop and so on)
Can I edit the character tags to make them custom ?
That unfortunately isn’t possible either, I suggest that several times way back. But didn’t find any agreement on that from devs. Now older and with much less enthusiasm I just keep fingers crossed that they not break something in MB, which unfortunately happens over the years.
As for example for this trick I discovered: Nice trick while browsing and replacing sounds through mediabay on sampler track or groove agent
I got explanation that “hacks and tricks” of this type is not in Steinberg manner so it’s removed (intentionally or not idk) lucky for me I found a better solution but unfortunately I cannot share it here since it doesn’t go along with “Steinberg manners” too and as I explained even before it was removed that Media bay and it’s ecosystem is vital for my workflow and production, so i am a bit scared to suggest you to put this on feature suggestions 
Oh, I misunderstood your question. I thought you wanted to know how to add additional choices for the Attribute (Stressful, OR Calm, OR Scary, etc.), not add multiple items onto the same Attribute (Stressful AND Calm, etc.)
@User131231 's comments are correct for your actual question.
You could however place multiple words separated by spaces into a Text Field and the Media Bay Search function would find them both.
For example if your Mood Field on an item was set to “stressful angry” then searching for “stressful” would find the item and so would “angry”