Hello [URL]   You will be ok doing that. You can install the new drives then use the iDRAC to create a new RAID5 array with them while still using existing RAID10 in production.
I have PowerEdge R740xd BIOS 2.2.11 (iDRAC not enabled yet) with PERC H730P. I have about ten HDD with ESXi hosting VMguests in production (RAID 10). If I turn add 8 additional drive > enable iDRAC > get in to  RAID configuration at the host >  Start initializing new eight HDDs (RAID 5) which may take 2-3 hours, [Important question] Can keep using my original HDD available and production servers while the new drives are being initialized? I understand I need to bring down ESXi for may be an hour to get in to RADI configuration. Thanks.
Add and initilize new HDD and keep existing in production - Dell R740xd PERC 
H730P