Oooooh - I'm experienced in that, we got the very first HP 5971A in the state. Disclaimer is that I'm not so computer literate, but we built our own custom libraries on it (revision A.03.04 if memory holds), libraries still in use today on newer Agilent softwares.
I'm retired now, so going from memory, but we created our own custom search libraries, had tow with over 2000 spectra each. If you don't know how to view The Stack, learn how, it helps.
So what we'd do is obtain the spectrum we wanted to add to library (often using average and subtracting baseline), that automatically goes into the "X register". In Data Analysis there is a Create Library command, which essentially makes a directory ABCDE.L, the L extension makes it recognizable as a Library. Then Select that Library. Then Add Spectrum. Note that a window will appear, and once you enter in the structure (if known) or a MW higher than actual (like 9999 for unknown), those two fields will be locked; all the other fields can be modified after the fact.
So even if you select a spectrum from a commercial library, that also goes into the X register, so you can also similarly load that spectrum into your own custom library.