Possible? Yes... fun or easy... no. Essentially you'd need to setup static routes for the traffic you want to go out over the dial up. It's not crazy hard, just is a lot of work and you'll have to add a route for each host / network you'd want to go over the dial up. You'd have to add the routes each time you connect / bring that network up, and you may need to do it dynamically (assuming your dialup default gateway may change each time you connect). You'd really need to write a batch file / script to pull it off, and depending on how many networks / hosts you need to connect to, this maybe really painful.