Setting a conditional variable

[mozart at pisem.net]

Hi everyone! I'm trying to set a conditional variable in my sql.conf. 
According to value of  %{NAS-Port-Type} my  variable (i.e. MYVAR) should 
be assigned to value returned by different programs through 'exec' 
module. I couldn't find any examples how to do it. So I will appreciate 
if someone helped me.
ps. I'm using version 1.1.2 from FreeBSD ports collection.