Can you make a SSS pickguard into HSS by liking cutting a bigger part out for humbucker where the 3rd single slot is? I'm thinking of doing it to a chrome one, but that might be too hard.
Yes, you can do it. However, you'll also need to make sure your guitar body has space to accomodate a humbucker, otherwise it will be pointless.

That could be done if you got the right tools, motivation and if you've got time

Just cut a larger part out of the third single slot and it's done
Be sure you'll make the right measurements and you'll be carefull with wires etc. underneath them but I'll suppose you're clever enough to savely pull those wires away
Simpler solution.

Buy new pickguard.

But like Denthul said, check the existing routing first.
Actually called Mark!

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