Yes you can freeze most greens like that. Put in a colander in the sink and pour boiling water over them. You might have to get a couple of forks and turn the heap over midway to make sure it is all wilted.
Rinse in cold water and squeeze excess water out. I have a couple of ice cube trays I use for this type of thing and I just pack the greens in and make frozen cubes. Once frozen you can decant to a bag.
Use for curries, soups or anything else you want greens for. The spicy ones are pretty good in pasta sauce or mac and cheese if you fancy a bit of a kick to them
