Mirror is RAID1 not RAID5. You need at least 3 disk for RAID5. FreeNAS uses ZFS for filesystem. One of the windows users might be able to help with reading it in windows 7.
If you are getting a SOHO NAS, you cannot add the drives directly and retain the data. However, migration is still possible without getting additional disks. To do this, look for a NAS which supports creating degraded RAID array or support raid migration (netgear/infrant calls this xraid). If it is not listed in the manual, call the manufacturer to verify before you buy.
1. Backup any important data you don't want to loose
2. Pull one of the drive from your freenas box
3. Add the pulled drive to your new SOHO NAS and create a degraded raid1 (this is a mirrored raid with 1 disk missing)
4. Copy the data over your network from the freenas box to the new SOHO NAS. Direct copy will be fastest, but you can also mount both NASes on your pc and copy the data (this will be slow)
5. Once the data is copied, pull the remaining drive from the freenas and add it to the SOHO NAS.
6. Add the next drive to the RAID array on the SOHO NAS and it should rebuild automatically.
7. Enjoy your new SOHO device.