Plastic water lines.
Chip, Your lines postdate mine in our E31 but I'd venture a strong guess that they are PEX (cross linked ethylene) and the simplest way I know to add fittings to them are with Cash-Acme Sharkbite fittings. Take a look at this video of theirs and you'll then know all you need to add any fitting to your system:
http://www.youtube.com/watch?v=POCKLxObPWs My experience with these truly magic fittings was on our vintage 1969 Airstream Caravel trailer. In a previous life, the copper lines were allowed to freeze resulting in a burst pipe in the toughest place you can imagine. I was facing removing the entire shower pan to get access to the pipe that ran under the threshold. It was about then that my neighbor, a recovering plumber, introduced me to PEX, and the plumbing supply folks, to Sharkbite. I was able to replace the offending line with two 90's in it remotely so to speak, at arms length by simply pressing a length of PEX into each fitting. If you choose to add valves using this method, the trip to the plumbing supply store will take longer than the installation. In actuality, it'll take longer to watch that video than it will for you to install both valves. Go for it, Glyn Judson, E31 hull #55, Marin del Rey, CA (no freezing worries here, EVER)