Currently, there’s no way to add your own objects to the built-in object library, but if you have custom objects or analysis groups which you have created and you find that you them use over and over again in different projects, you can create your own repository for these objects.
One way is to place all of the structures or analysis groups into a single reference simulation file. Then whenever you want to add one of them you can open up the reference file and copy and paste the desired object into your current project file.
Alternatively, if you prefer building the simulation file from a script, you can maintain a folder with script files where each script file contains the commands to build a certain object. You can then call the script file whenever you want to add a particular object, similarly to the example on the creating your own script commands page.