RAID

There already a RAID thread going, but I don't like jacking peoples threads. 
If I were to start a RAID with my 250 now, could I just add on another drive later on, or would I have to start over anew with both drives?

If I were to start a RAID with my 250 now, could I just add on another drive later on, or would I have to start over anew with both drives?