Hi. The folding mirrors were an option for the SLK but only for the Japanese market. It's possible to retrofit them but it takes some time and quite some money. You need to remove both mirrors, door panels, under the driver/passenger and middle console. The parts: both internals for the mirrors with the motors (from the dealer), the switch for the middle console, the plastic trim around it and the cable (or make one, like I did). I've done this 6 years ago and it cost me roughly 750 euros. 350 for each mirror, 35 for the switch, maybe 5 or 10 for the plastic trim and some extra for the home made cable. If you buy the cable it turns things much easier. The way I did it was much harder since I had to cut the socket off (middle console) and rewire the new switch. It's not straightforward since I had to look at the wiring diagrams to do it. Plus, you need to run 2 wires for each mirrors (for the motors).
Possible? Yes. Worth it nowadays? Probably not, unless you can find 2nd hand parts pretty cheap (good luck with that) and you are up for a challenge.