Yes, you can create softraid arrays at any time. FAQ 14 will be your friend. Note these additional comments regarding RAID, from FAQ 14:
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Some words on RAID in general:
- Before implementing any RAID solution, understand what it will and will not do for you. It is not a replacement for a good backup strategy. It will not keep your system running through every hardware failure. It may not keep your system running through a simple disk failure. In the case of software RAID, it won't guarantee the ability to boot from the surviving drive if your computer could not otherwise do so.
- Before going into production, you must understand how you use your RAID solution to recover from failures. The time to do this is BEFORE your system has had a failure event. Poorly implemented RAID will often cause more down time than it will prevent. This is even more true if it has caused you to become complacent on your backups or other disaster planning.
- The bigger your RAIDed partitions are, the longer it will take to recover from an "event". In other words, this is an especially bad time to allocate all of your cheap 500G drives just because they are there. Remirroring 500G drives takes a much longer time than mirroring the 4G that you actually use. One advantage of software mirroring is one can control how much of those "huge" drives is actually used in a RAID set.
- There is a reflex to try to RAID as much of your system as possible. Mirroring your boot partition in a software RAID system is difficult, and often pointless, as most PC hardware has difficulty booting from any drive other than its primary boot drive. Even hardware which CAN boot from other drives will often have difficulty determining when a drive has failed to avoid booting from it. OpenBSD's "altroot" system can actually be BETTER for some applications, as it provides a copy of old configuration information in case a change does not work quite as intended.
- RAID provides redundancy only for the disk system. Many applications need more redundancy than just the disks, and for some applications, RAID can be just added complication, rather than a real benefit. An example of this is a CARP'd set of firewalls provide complete fail over redundancy. In this case, adding RAID (either via hardware or softraid) is just added complication.
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Where I use boot-on-RAID and root-on-RAID -- note that they're not exactly the same thing -- I always confirm that I can control via BIOS configuration which physical drive is used for booting.