Add drives to RAID?

Question

RAID

Hello!
There is a server whose disk system looks like

I understand that RAID 5 is created from 3 HDDs (1,2,4), they are marked as dynamic. Disk 0 is allocated for the system, part of its space is unallocated, space on disk 3 is allocated only for system files (but how did they get there?)
Question: what about disks 0 and 3? Mark their unallocated space, and then what? Is it possible to add them to the existing array or is it unrealistic?
Thanks in advance!