Someone in the Baja forum did this sometime back.
A couple things...
Mercury sells two kits to do this. There are two different threads that your oil pan can be and you need to figure this out before/as you order. You can do a little research with your local Merc shop, or www.bammarine.com should be able to do it. I think they are about $65 bucks each.
You do not have to permanently marry the oil drain hose end(s) to the bilge plugs. I keep my oil drain tubes pinned up near the oil filters to I can keep an eye on them. When it comes time to drain the oil, I have a coat hanger that I straightened out, and fish it into the bilge toward the front of the engine compartment, manually hook it to the oil drain tube(s), and pull them out the back. Then reverse the process when finished.
I find that leaving the hoses on the bilge "floor" all the time allows more "stuff" to accumulate down there and stay dirty. They also restrict flow out of your bilge plug(s) if you flush/wash your bilge a lot.
If you have a twin engine boat, with a center stringer, your builder should have installed TWO drain plugs on either side of the main stringer. I have seen SOME builders notch the back of the center stringer, and install only one bilge drain in/under this notch. IF your boat is like the latter, you will probably want to install two more bilge drains and marry them to the new drain plugs. Otherwise, it will be very difficult to extend the oil drain hoses out the bilge plug hole if it needs to go around an obstruction (the center stringer).