If you only want to safe the NEF as JPG, once you've got them selected go to File and Save As choosing the file format you desire.
If you want to apply other changes to the files, you could use this to save these to a new folder as NEFs and then run a batch process on the entire folder, e.g. converting Adobe RGB to sRGB, sharpening, and then save as JPG. You'd need to set up your changes in settings files and choose which ones to run as a batch.
There is a Watch Folder option that will run a batch on all files in that folder immediately. I've never used this, but I imagine you could set this up, save your files to the Watch Folder and then the batch setting of your choice would run automatically.
NX2 is not the speediest at batch processing. I use Photo Mechanic for conversion to JPGs via the 'save as' option on selected or filtered files. It uses the embedded JPG in the NEF file and it's very fast, provides sizing and conversion to sRGB, watermarking, etc.