question

You can by default add your model user commands to a user library either to be installed in your model or drag dropped into the model:

However, at the moment if you add a command twice you can get two versions in the user library, so you'll need to manually prune the tree of the old one.

You can set FlexSim to automatically load a user library so you always have access to them from the global preferences.

You can also put the commands under a custom node of the user library and create the modellibraries structure to refer to those commands when you load the library or create a new model (using the dropscript method Felix mentions). Once under a custom node they will probably not be editable from the UserCommands editor and so you'll likely need to edit the tree to update your shared commands.

To manage a shared user library of commands you'd probably want to save it as xml and merge any changes in a version control system. To make this work with the least manual merge intervention, you'd could sort the node that holds all the user commands alphabetically, or create a .ffm map file that splits out each user command as its own xml. That requires creating a dynamic .ffm which could be part of a scripted custom button you press to save the commands out. At the moment I don't think there's a standard user library pre-save hook for you to do that automatically every time you save the library.