The reason you can’t add the disk is because a RAID1 volume can only have a maximum of 2 disks in the array. That is why you are being presented with the error, you can only add the disk to the array if you are going to change the RAID level type (to RAID0 or RAID5 for instance).
Here is how I would recommend doing this, pay very close attention to details or you will lose data. Also before starting, you must make sure that the size of disk (GB) A and B are at least as big as disk C or D or this process will not work.
You have a RAID1 with disks A,B (let’s call the array oldWin7) that are Enterprise Disks with old install and all the data files you want.
You also have a RAID1 with disks C,D (let’s call the array Win7SP1) that are non Enterprise disks but have clean install.
You want the image from disk C/D to be on disk A, then you want to manually copy data from disk B over to Disk A so it will have new image and old data files.
In order to do this, you will need to degrade both volumes.