Willy, Essentially what they do is create a sandwich of sorts in the mast where they want the dam using pieces of foam that are fitted perfectly to the inside of the spar. They drill the drain holes and insert clevis pins which will be removed later. The top foam piece of the sandwich is pushed up to the drain hole clevis pins. They drill a couple of fill holes and using a Sikaflex type of material, fill the area between the foam pieces thereby creating a dam.
I really can't see any way of accomplishing this with the mast stepped.
But if you decide to pull the stick you could probably make it work then.
You mentioned that your mast is a Selden. If you look at the base of the mast and copy down the engraved number/letters there, you could contact Selden and they can pretty much tell you everything about it.
Frankly, I am somewhat surprised that it doesn't have a dam in it already.
Please don't make the mistake of squirting that expandable foam stuff in though. You or someone along the line will surely regret it.
Better to live with a little water in your bilge.
If you see corrosion developing around the base of the mast or step, spray it with some kind of corrosion block or something.