Odd request, but is there any way to buy a game on one's PSN account that they've already "bought" as a free game through PSN+?

Regardless of whether the game was free or purchased normally though PSN, any game registers as purchased, making it impossible to buy the game more than once on the same PSN ID. 

I want to be able to add certain games I've already received through PSN+, but without the expiration date even though I'll likely be keeping the PSN+ subscription for as long as I use the PS4 for gaming. 

Make another account and buy it there.

I've done that before on sub accounts. The only problem is the PSV only allows for one account/SN per device without jumping through hoops to make it work (in all fairness, they are individual devices, but that doesn't help).

So when you buy games on your non-PSN+ sub account those games won't be playable on your main account/PSV in a nutshell.

It's preferable to have everything on one account (your main account). I already have a small collection of orphaned games that were purchased on other accounts that I don't have open access to.

I think your sub has to expire before you're given the option to purchase a game you got free on PS+.

I believe this is correct, I even purchased a code for a digital copy of a game to test this out. Though when I tried to redeem it in the PSN it gave me a message saying I already purchased the game so I doubt it is possible.

I'm guessing that because it's a fairly rare instance when someone would want to buy a game that was already given to them for free through PSN+, there is no direct way of doing this, hence the thread. I figured someone may have known a workaround without using sub accounts.

I've heard you can still buy these games. Just need to contact Sony customer support and tell them to get the game out of your bought list.

I was going to say "Just wait until PS+ expires and then buy it", but you aren't going to let PS+ expire......so.......why are you trying to do this?

to buy games which go on sale some times for joke prices so you would want to buy them to ensure you have the games forever, or simply to buy a game you got free on psn to support the developer more directly?

Ah, I didn't think abour sales, that is a good reason.