Do the mags from the Swedish K and Soumi M31 interchange?
I'm going to presume you're refering to the Suomi drum mags.
An original Swedish K with the removable mag well will not allow the drums to fit. The removable mag well was only intended to allow the "coffin" 50 round magazines to fit.
If you have a K "tube gun" then it might have already had the receiver walls trimed to allow the drums to fit. If the well in your tube is retangular it may allready be cut to accecpt the drum. You typically have to either hand fit the drums or trim the receiver walls if you want to use the drums in a K.
36rd stick mags will fit in either.