Hi Sabry75 If you have Open Manage Server Administrator installed, you can do the merging from there. It is called reconstruct. You may refer to the OMSA user guide below, look under Reconfiguring/Migrating Virtual Disks:-  <ADMIN NOTE: Broken link has been removed from this post by Dell> Let us know how it goes.
I have Dell Powerdege 2900 server and I need to merge new hard drive in its RAID 5
how to Add new Hard disk to RAID 5 on 