Will I be able to add storage down the road ?

I'm a complete beginner, I got myself an hp elitedesk 800 g3, it came with a 250 gb ssd and i also bought a 480 gb ssd to add to the system. For what I've seen online people tell to separate the drive where the os lives from the storage drive (that's what I want to do), but if I do this I will not be able to configure a raid 1 as I have olny one storage drive at the moment, so I was wondering how I could configure the server in order to add another 480 ssd and make a raid 1 in a month or so, and how would I do it.