It's perfectly doable, but the common distros do not include any kind of reasonable front-end software to issue the lower-level RAID commands for you. If you study the manpage for mdadm, then you can work them out for yourself, but there's still some silliness with /sys that's not at all obvious to most folks.

I have written RAID-1 auto-repair scripts (for business clients) that truly make this a simple matter of unplug old drive and plug-in new drive. Works great.

But this is not something that most distro's would want, because it could easily result in somebody's USB/eSATA drive getting erased on insertion and made into a RAID member instead of whatever the user might have intended. That's why it's not an automatic thing.

EDIT: RAID appliances generally do have automation like that, but the scripts for it are often proprietary to the vendor.

Cheers