No, cabins are not available in economy class but if you have all 6 seats, you can cover it with a bed sheet and it will be as good as a cabin
Below is the pic, we did the same last winter (this sheet is smaller in size, you can use a bigger one to cover fully)..
P.s. Some coaches of Night coach express have curtains also which you can close to get a cabin like feel but remember its not installed in all coaches (so its ur luck if they are available).