sunscreen, source port

I can't find a way to specify a service in sunscreen
which is defined by the source port and not the
destination port.

I want to allow a connection originating from
a specific source IP and source port.
From what I've seen the port definition
in 'add service' is for destination port only.

I've tried both FORWARD and REVERSE but without any luck.
Probably REVERSE does something else that i do not
understand.

Any help would be appreciated.
thanx

Kapetanakis Giannis