I think you are referring to an ArchiCAD Project Archive or PLA. Not PLN.
Loading a PLA as a library is one way to load library parts into a project. It's the parts loaded into that PLA that are used when that PLA is loaded, not the actual modeled elements that are contained in it. Because all the parts are loaded into the PLA, you are able to open it and edit easily enough, but it can be a bit of a nuisance resaving it. If you read the elements directly from the archive, you are effectively loading the PLA as the library, so when you then go to save it, ArchiCAD says that the file is already in use, which means you have to save it as a different file.
Alternatively, when you open the PLA to edit it, you can extract the parts to a folder and resave the PLA. Once it is saved, you can delete the folder containing the parts, and simply load the PLA as a library again.
Consider using a Library Container File (LCF) instead of saving and loading PLAs. Not only are LCF's much smaller (if you choose to compress them), but they are also easier to create and manage. You don't even really need a working file (like the PLA), as you can create an LCF from any file.
So, in summary, starting out on this adventure, you may find it easier to use LCF, and they'll be quicker to load, especially over a network. But having said all that, if it's only a couple of objects you're using in a Project Specific Library or even an Office Library, you could probably save the parts into a folder and load the whole folder. As it develops and grows in size, you could start saving it as an LCF to speed load times.
Just my 2 cents...
Cheers,
Link.