to make the picture more grainy,
1) there has to be grain in the first place - if your hdr processing is good there will not be grain, honestly
2) secondly, if you do too much sharpening, then yes, the grain will become super obvious, otherwise no issue
last night i went on some maniac spree to redo a lot of old photos that were hdred.. still not natural enough. think i'll only use it very very very sparingly in the future.