With appropriate 'route' statements? You will need an inside router to route the traffic -to-, though.
I am not sure that I understand your question. "through" could mean movement in either direction, so I am not sure whether
172.26.50.0 and 10.56.56.0 are "inside" or "outside" of your PIX?
Are you trying to have inside 172.26.50.0 traffic be sent to your PIX inside interface, with your PIX inside interface then sending the traffic back inside to 10.56.56.0 ? If so then with the PIX 515e you would need to use PIX 6.3 or later and use 802.1Q VLANs -- or with the PIX 515e you could use PIX 7.0 or later with one of the features that is new in 7.0 (but which, if I understand correctly, would require that at least one of the two networks be reached by a VPN.)