You can edit the fpc.cfg file located in your fpc installation directory, and put any global search paths using -FuPATH (for units)and -FiPATH for include files. You can also put an asterisk * at the end of the path to make fpc look into all subdirectories of that path. I think that is the only 'global' way.

There are some issues with this approach though, identifier completion does not work as it should as Lazarus can not check the directories specified in the fpc.cfg file. Also when debugging Lazarus may ask you to find the required sources.

Note that you can save Lazarus compiler options to a file. You can make a template project, list all the paths of units in the options and save this to a file, and then load it for each project you need.