Can you short-sell stocks that you actually own?
No, unless on another account.
Although it makes zero practical sense.
That’s called short against the box. Completely legitimate and should be the only lawful short sale.
There was a time when short selling what you owned was allowed as a tax strategy. “A short sell against the box is the act of short selling securities that you already own, but without closing out the existing long position. This results in a neutral position where all gains in a stock are equal to the losses and net to zero. The purpose is to avoid realizing capital gains from a sale to close, and so it has been restricted by regulators in practice.”
No... a short sell by definition is borrowing a share from the brokerage (who takes it from another account or holds a certain amount in a central account) and selling it at the current market price. Then when you close you have to buy a share to give back to the brokerage.
If you own the share you can sell it out right expecting it to drop and then buy back in - same idea, just a different method.
I believe you can structure a Covered Call that acts like a put.
https://en.wikipedia.org/wiki/Covered_call