Does the DC-3 passenger airliner have slats?
Although it's technically possible, you aren't going to find slats retrofitted to a DC-3 (nobody would ever pay for it). 

But yes, in theory you could improve the DC-3's stall behaviour and single engine performance very cheaply with Vortex Generators which can be thought of as a "poor man's slat".  VGs increase stalling AOA by between 1/3 to 1/2 of what you get with a slat (say, about 18-20 deg, vs 25 with slats) depending on the airfoil.   

VGs have huge benefits on light twins, reducing stall speed, taming stall behaviour, improving aileron response, and often reducing Vmc to the stall when installed on the fin.  They are quite miraculous, although they can be a headache because they are usually glued on and if the kid you pay 20 bucks to wash your plane knocks off too many, you are grounded until you fix them.

I can't find anybody that sells a VG kit for the '3 although it does seem to be possible to install them on a wing with boots. Possibly the DC-3's airfoil requires VG placement far enough forward that the boots are in the way (each airfoil has a chord wise sweet spot and if they are too far aft they don't do anything).  Or maybe nobody has thought of doing it yet.