a pbi package is a container which contains all dependencies freeradius2 needs. this prevents conflicts between other packages with different versions like perl or something else.
The problem with freeradius2 will be that the package is compiled with different options like LDAP, kerberos, mysql, and so on. So you probably need to recompile them first.
Further pfsense 2.0.x uses freebsd 8.1 and pfsense 2.1 uses freebsd 8.3. So there is no way around to recompile everything on 8.3 basis.
Hope this will help you to understand