You can configure networking per interface by editing /etc/hostname.XXX file where XXX is your interface name (em0/iwn0/…)
Example /etc/hostname.iwn0:

nwid YourNiceSSID
wpakey WPAkeyYouHave
dhcp

This will allow you to connect to the network YourNiceSSID simply by

sh /etc/netstart iwn0

Example:

 ifconfig iwn0 nwid "YourNiceSSID" wpakey "WPAkeyYouHave"

Is equivalent to the above /etc/hostname.iwn0 example