QFN 0.4mm Pitch

Hi 

I am designing a 0.4mm pitch 32 pin QFN Package(.dra). So far i have used only 0.5mm pitch packages.

I read that for these fine pitch packages solder mask needs to be designed which looks like this on the right:

So question is: this large solder mask opening should be defined in the .pad file or

Should it be done in the .dra file and if yes, what is the common procedure for this 

Thanks