Yes, you should be able to add to a libary, example, when you create new tests in your target folders they will be added to the library, I think you may be confusing terminology a bit, more tests can be added to a library after it is created but you cannot add entities to a baseline after it has been taken. Each time you create a baseline it only has the entities as they were at the time that baseline ''snapshot'' was taken, in order to add entities to a baseline you have to create a new one, I will explain in detail below.
So basically at a high level it looks like this. In ALM you have a folder of tests that you would like to send or ''share'' with another project. So the first step would be to create a ''Library'' a library is a collection of objects, tests, requirements etc that you in a sense are ''cataloging'' in the library so that you can keep track of changes, an example would be a test is added to a folder that you have selected in the library or a test is updated. Once you create a Library you then create a ''Baseline'' which is a ''snapshot in time'' of those library objects.
So let's say this is my scenario. I have a test folder called ''Jenkins'' in my test plan on project A, I want to send the contents of that Jenkins folder to project B, I would go in and create a Library and put a check mark by the Jenkins folder under tests in Project A. After I have made the library then I can create a ''baseline'' based off of that library in Project A. Then I go to Project B and import that baseline and place it under the root folder of my test plan. This brings over the current copy of those tests under the ''Jenkins'' folder as they were at the moment in time when my baseline was taken in Project A.
So let's say I add tests under the ''Jenkins'' folder on Project A and I want to bring those additional tests over to Project B how would I handle that? Well first I would need to create a new baseline in Project A a new ''snapshot in time'' of how the entities in the ''Jenkins'' folder currently appear, this new baseline would pick up the extra tests that I added in the ''Jenkins'' folder. Once I created the new baseline in Project A I can switch to Project B, in project B I will open the Libraries module and right click the library that I want to update with the new tests that I added. After right clicking the library I select ''Compare To'' then I click ''select baseline'' then I select the new baseline that I created in Project A. The comparison should show the tests that I added when I created the new baseline in Project A and compare them to what is currently in the library for Project B, then I would click the ''synchronize'' button at the top of the dialog, this updates the entities in Project B's library (and test plan) with the ones in the new baseline from Project A.
The best way to understand all this is going to be to setup two test projects in ALM for the purpose of testing how baselining works, I always recommend customers set this up to familiarize themselves with the process before they try on production projects.