Earlier or later (usually earlier) you'll find out that your camera/lens is not in the database. Well, that's easy to fix, just add it to the database yourself! ;)
As stated in the section How database files are found and loaded, the library will look for xml database files in various subdirectories. The best place to put your own definitions into is ~/.local/share/lensfun/. This directory is guaranteed to not be overwriten by library or even OS upgrades, so it's safe to keep them there.
So now you just create a new file under ~/.local/share/lensfun/ – with any name, but with the '.xml' extension:
<lensdatabase version="1"> ... put your definitions here ... </lensdatabase>
and it will be automatically found and loaded at library initialization time,
Of course, if you create new camera/lens definitions, I will be grateful if you send them to me so that I can include them in future releases of the database.