Turkish airlines date change during layover

I have done exactly what you're suggesting (although in my case it was to catch an earlier flight - but same concept), and whilst I was ultimately successful it was one of the worse customer service experiences with any airline I've ever had.

I ended up being told to go to about 5 different customer service desks airside at Istanbul airport (including the transfer desk, both service desks in the lounge, and several others - with each telling me to go somewhere else) before I finally went through immigration and to the ticket counter who was able to make the change - but not before charging me US$30 even though I was on a completely flexible business class ticket.

If your ticket did not allow for changes (as mine did) then I suspect it would have been a very different story and would not have been allowed. Also, if you had checked bags then there is zero chance that this would be successful as your bags would be checked through to the final destination.

What you are suggesting is something that I would not suggest with most any airline, but that is especially true with Turkish Airlines who just simply don't handle this type of thing well (and I say that as a regular flyer of TK!)

Also note that if you ticket is booked via a corporate travel agency, they will be notified of this change - even when it's made at the airport on the day of flight.

This might work, but it will likely be rather costly. The terms will depend on the fare rules that apply to your ticket, so you'd need to check those carefully before you purchase (the Turkish Airlines website will display them when you search for flights). You'd generally have to pay a change fee plus the difference in fare. The difference in fare could be substantial, possibly even more than the ticket cost in the first place, since you'd be adding a stopover and changing from an advance booking to a more short notice one.

It also could be difficult to wait to do this when you arrive in Istanbul, since the entire system is designed for you to get on that connection, and trying to change that and get your checks bags back shortly before departure could pose a problem. If you're going to change your ticket to add a stopover, I'd do it well in advance, not when you get to Istanbul.

It would really be easier and cheaper to get the company to reimburse the price of a ticket without the stopover, and then just book a ticket with your desired stopover from the start. Whether this is allowed by their travel and expense policies is a different matter.

I would be careful about the one way ticket though. If you're a visitor to South Africa, the airline or the immigration authorities might require proof of onward travel. I'd make sure to have another ticket out of South Africa if this is the case. Also keep in mind that one way tickets sometimes cost more than round trips.

Instead of trying to do a date change on arrival, which will vary from expensive to impossible, I would advise you to find a fare that allows stopovers and expense that to the company up front.

If you check the small print of fares, you can usually find out whether they allow stopovers and, if so, how much they add to the cost. ITA Matrix makes searching & viewing these detailed fare T&C's fairly easy.

The sane approach is this to identify one of these fares where stopovers are allowed, ask the company to approve the cost (as a simple one-way), then book it with a two-week stopover added.

The sneaky approach is to do the same and actually book the fare as a simple one-way (so company policy is fully followed), then call up the airline and change to have a stopover. This may incur change fees though.

Both assume that your company is concerned only about cost. If they have the kind of vindictive, raisin-pooping bean counters who will ding you for Violating Policy even if the cost to the company is zero, you're better off avoiding either approach.