Mike,
It sounds like what you've got is the "spindle-lube" spindles, and are asking about changing to a "Bearing buddy"-type system?
My advice is don't do it. The bearing buddies maintain a slight positive pressure on the bearings, which is a good thing if they are properly maintained. Properly maintained means fill them properly and resist the urge to over-maintain and over fill them, which will result in blown seals. It's just hard to resist adding a little more grease for good measure, and too much is a bad thing. Maintenance involves disassembling everything at reasonable intervals to make sure you don't have water intrusion. It's not common, but you can't tell for sure without taking them apart and re-packing the bearings.
The last two trailers I've had have had the spindle-lube system. From my experience they aren't any more likely to allow water into the bearings, sometimes just a bit just inside the rubber cap, but never at the bearings. The great thing about them is that you can make sure there is no water at the bearings by pumping in fresh gease and completely flushing the old grease out. Any water will be visible in the old grease that comes out of the hub, and nothing has to come apart but the rubber plug.
For me, the annual re-packing of bearings has been replaced with a few minutes of pumping fresh grease into each hub. Much easier, and I'm more confident in my trailer bearings.
If you're determined to use the bearing buddies, you should be able to simply remove the grease fittings from the spindles and replace the caps with the rubber plugs with the bearing buddies, which can be bought at just about any marine supply store. Make sure you know the diameter of the cap you remove, there are different size bearing buddy caps. I typically get my trailer supplies from
www.championtrailers.com, but Eastern Marine is a sponsor here and they seem to have a good rep.
Bill
Looking for trailer advice!! 
