I am not quite sure to my understanding of your problem.  So let me see if I can try and help.

1st Yes loading lights from the library will delete all of your preexisting lights and install the new ones. 

2nd - This includes any animation of those previous lights and if the new library lights have animation, it will install that.

3rd  Attempting to save the previous lighting as a light prop will work but then when you click on it, it again will delete what you have. 

4th  Yes the more times you click the add light button the more lights it will add.  With Poser you will only be able to preview 7 or 8 of these lights, though the actual number in the render are limitless.

5th.  If you wish to use lights from the library with your lights, then you will have to use pencil and paper to write down all of the information of those lights that you want to keep or add, and enter it manually.  The three lights that poser uses when starting a scene are basicly set to light the entire scene and really are not the best used in the final render.   They are really just there so you can see what you are doing while setting up your scene.

What I would recomend is to find the lights from the library you want to use, add them to the scene then add your own lights to suplement the prop.  You can animate those library props the same as your own.  If you find it impossible to do so, just delete the keyframes to those lights.  This is only if you do wish to venture forth and learn lighting.  Lighting, a very key component, but yet a very hard thing to learn.  Some of us are not gifted.

There is a possibility that I have overlooked something and someone will tell me I'm wrong, but as of right now, I don't know of anyway to keep existing lights and lights from the library.

Oh yes there is one thing I forgot, I believe you can save the animation of lights as a pose, well in theory anyways.  Haven't really done it for each scene I make has an animation taylored to that scene.