Can an adjective be converted into a noun by '-s'?
In fact, yes; good point. You can turn many adjectives into pluralized nouns:
The yellows (the team in yellow t-shirts)
The dispassionates (phlegmatic people)
Your smarts (your know-how)
Hello, my pretties (a pimp's greeting, I suppose)
Put your briefs in the hamper (underpants)
The indolents (the rentier class)
Some you have to do additional work on:
Fasties, wiseguys, etc.