Can I add butter to yeast dough which has already risen?

No, you shouldn't be adding butter at this stage, it will take too much kneading and undo the rising.

Whatever it is you were baking, just go ahead and bake it this way. There are many breads which don't use any fat at all. The taste will be different than with butter, but it will still be a good bread.

Have just done it in error before reading up on it. It will change your bread so you will get a buttery croissant feel to the outside of the roll, plus your bread may turn out a bit ugly. I personally liked it.