What sort of raid array do you want? e.g. you can't create a RAIDZ/5 array with 2 disks that will hold more that will fit on a single disk.
The best you could manages with just 2 of the disks is RAID0 (striped only), then add the old pair as a mirror of the first 2 which will potentially give you better performance (reads/writes split across 2 drives) and redundancy to loose 1 disk out of the mirror pairs. But no extra storage as you would still have a max of 2 x disk size.