One of my players is a sorcerer that happened to befriend a NPC who taught him one spell. This player already has all the spells that a lvl 6 sorcerer should have, which makes the "learn" option on the manage spell list grayed. So far we haven't found a way to add this extra spell. Is there an "easy" way to do this without having to create homebrew content?

Create a homebrew feat that gives them that spell and then give it to them using the Manage Feats button on their character sheet.

Is there any tutorial on how to do this? I can0t find a way to make it so it simply learns the spell, either it has unlimited uses or a fixed amount of cast per day (unrelated to spell slots).

in the spell option, click "Counts Against Learned Spells" - no, Consumes Spell Slot - Yes

I can't find any of those options on the spell menu:

weird, because I show it when I'm modifying it in subclasses (which is how I discovered those fields). ...uhh... not sure then

Quote from DevanAvalon >>

weird, because I show it when I'm modifying it in subclasses (which is how I discovered those fields). ...uhh... not sure then

Those fields are correct in the subclass class features add spell screen, but not in the feat add spell screen

eleuis: you're limited to the NUMBER OF USES and RESET TYPE fields to limit how many times they can use the spell you're giving them. Or create a subclass for them that adds the spell. If you choose the latter, DevanAvalon is correct