You never put a filesystem on a disk that will be in a raid array. The filesystem is created on top of the array once it is created. WIpe the disks and you should be able to add them to an array (unless they are usb disks).
You do realize that raid 0 will lose all data if one drive fails?
The answer is a bit complicated for my level of knowledge, but I understand that you have to create the raid before installing the system...
I don't understand your response. You created the filesystem before the array. You need create the array before the filesystem. Does that make more sense?
Can I install OMV on one hard drive, then add 4 other hard drives and then create a raid 0 with these 4 other hard drives ?
Yes.
When you add the 4 other hard drives, you must wipe them in the Physical Disks tab in OMV. Then you will be able to create a raid 0 array from them. Once the array is created, you can create an ext4 filesystem on the new raid array.
It works! Great! Thanks 
Dany
Hello, I have installed and mounted 4 additional hard disks on my OMV7 NAS server. I would like to combine them into a single virtual disk to have a large space available for data that I will only keep for a short time.
I have created EXT4 file systems for each disk and now I would like to create a RAID 0 (or LINEAR).
But when I click on create, it doesn't offer me any disks...
What should I do, please?
Maybe you should consider a mergerfs pool instead of a Raid. https://wiki.omv-extras.org/do…mv6:omv6_plugins:mergerfs
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