Thank you very much for the responses!
It seems that I need to have a deep thinking on what I need and what I want... and to meditate on the difference between RAID and backup.
This is a good opportunity to do it!
Indeed I used the admin login, I will try with root one.
The main reason to add the two new disks is that my NAS is now full 
When I created the NAS last year I wanted to test the OMV solution (free solution, large community of users) because I had reached limitations with my Synology NAS and I decided to start with 3 disks (budget).
I chose WD Red 3TB, same reference for the 6 disks.
I use it mainly for media center purpose (Plex Media Server) and for some data storage (personal videos for example).
I understand now that this is not the best option for the second one, and I will think about finding a way to secure these data on an alternative solution. I will save the important data on something else.
Concerning adding disks to the RAID array, if I remember well I did it once last year to add the fourth disk and it worked perfectly.
I must have used an option in the OMV software.
I don't understand why it is not easily feasible this time.
I think I cannot restart from scratch without losing all my media center data because I have no other disk capacity to welcome these 9TB data.
I will update the post when I am ready to progress on this.
Thank you again for helping!
Have a nice Sunday.
