wmaruya,
Typically, you don't need to add a route for the remote private subnet, because the default route on the pix covers that. In case you have a route added in your pix towards the private side of your pix which covers the remote private networks as well, then in that case, you do need to add a route on the pix. Make sure that you add a route and point to the outside interface of the pix
Hope that helps
Jazib