Yes and the handbook has a fine chapter on adding new disks.
The process basically amounts to shutdown (needed on regular PCs), install the disk, boot FreeBSD, fdisk it, bsdlabel it, newfs it, add it to /etc/fstab. It is quite simple especially on FreeBSD.
If you want to move your current installation to the new disk I would suggest a boot into single user mode, mount the afore mentioned and installed second drive and do a dump/restore from old drive to new drive; porting the fstab file as necessary and then shut down and physically swap the disks.
My OpenBSD machine was installed on a single 8GB disk. Later I hooked up an 80GB disk with two partitions in the slice which mounts as srv on / and local on /usr.
Code:
Terry@vectra-$ df -h
Filesystem Size Used Avail Capacity Mounted on
/dev/wd0a 147M 31.2M 109M 22% /
/dev/wd0h 393M 936K 372M 0% /home
/dev/wd0d 98.3M 226K 93.2M 0% /tmp
/dev/wd0g 6.7G 738M 5.6G 11% /usr
/dev/wd0e 148M 32.7M 108M 23% /var
/dev/wd1a 11.8G 44.2M 11.2G 0% /usr/local
/dev/wd1d 44.3G 12.2G 29.8G 29% /srv
Terry@vectra-$