You can, but macarons need to mature for at least 24 hours in the refrigerator for the best taste and flavor. In a pinch, I have done the following method to speed along the maturing process for macarons.  Take a small amount of heavy cream or milk and brush it on the inside of the shells.  This does not need to be a lot, just lightly brush it on the shells (you don’t want super soggy shells and you want the filling of the shells to do most of the maturing process).  Then proceed to fill your macarons as usual. Let the shells mature at room temperature for a few hours.  I find they are usually mature within about 4 – 5 hours.  Again, letting them mature in the refrigerator is best, so use this method cautiously.