You sure can. I'll assume your raid drives are hdb and hdc, and that hdb is your current drive with content already on it.
1) partition hdc as one big partition as a raid parition
2) make a file system on this partition
(1 and 2 are idential to the steps for initial install)
3) mount hdc1, copy contents from hdb1 to hdc1
4) repeat 1 and 2 on hdb
5) write you /etc/raidtab file that describes your setup
6) umount hdc1
7) mount the raid array md0 (or what ever you call it in step 5)
now, your raid-checking procedure will kick in and build a mirror of hdc1 onto hdb1. This will take a while.
If you're nervous about trying this with production data, build hdc1 and hdc2, and mimic the above procedure on these dummy partitions before you start with the real deal...