What exactly are you trying to do?

1. Replace the two M.2 drives with a new M.2 500 drive?
It is possible to replace both existing M.2 drives from RAID 0 (slot 2 & 3) into your new single M.2 drive (slot 2 or 3). Both slot 2 & 3 are NVMe and SATA capable but for RAID should be same variant (SATA or NVMe) and same size.

2. Add the M.2 500 drive to slot 1 of your laptop?
Please reference page 87 of your users manual (slot 1 is for M.2 SATA only drives), your 970 Evo Plus is PCIe NVMe and wont work.

3. Add the new M.2 drive to the RAID O?

The 970 Evo Plus drive can not be used in slot one.

Please explain further.