RAID should be transparent to your applications and OS. It's presented as a single volume to your OS, so it won't know the difference (unless you're doing something silly like using software RAID or fakeRAID).
And yes, it is possible to add disks to a RAID5 array. Whether it's possible with your array will depend, specifically on the capabilities of the specific RAID card you're using. If you can post that detail, I can probably help you out.
On the other hand, RAID5 with many disks or large disks is a horrendously bad idea (almost certain to get a read error on the single parity bit, rendering your array useless), so I'd probably advise against doing this. For the general case, at a minimum, I'd want RAID6, and generally prefer RAID10. And for reasons I hope are obvious, I also prefer to have my OS on a different array or disk from my data partition. Makes it a lot easier if I need to change things up later, like converting a RAID5 array to a RAID6 or 10... for example.
And, as pointed out in the comment below (thanks Hennes!), selecting an appropriate RAID level for your databases is much more involved than just slapping it on whatever the OS has. (Another argument for segregating OS and data arrays.) The RAID level you select will impact database performance, and what you want/need to optimize your database for (say fast read access, or large numbers of small writes, or small numbers of large writes, etc.) should impact which RAID level you elect for the array your database sits on. The SF "canonical" answer on RAID levels has more information (thanks again Hennes) on the advantages and disadvantages of the common/standard RAID levels, and should probably be a next stop for you.