Hey Pontus,
It's pretty simple to do. Make a tranform node that scales in y by .5 and then set the filter type to impulse. Now connect a second transform node to the first one and scale in y by 2 and set the filter type to cubic. Thats it! ...Almost, if you're running a newer version of nuke you might have to put some node inbetween the two transforms. I usually use a blur node that has no effect (set to zero). This will prevent nuke from thinking it's smart and trying to save on render time by concatoneting two adjoining transform nodes into one operation. Which is bad because it ignores the first ones filter type! Hope that helps.
Jason