Simple glaze
Yes you can thin out some icings, not something like butter icing however to rich. Add more cream, or milk over low heat, or as you said heat store bought, run out of time icing... However what you will end up with will be a thin layer of icing as it sets, not the glaze like consistency you seek... Try this good for all things yummy.. Take 2 cups 100% fruit juice, and add 1 cup of confectioners suger and the zest of one lemon lime or orange. Bring to a rolling boil over medium to high heat. Reduce heat and let simmer until the mix coats the back of your wooden spoon. You should be able to run your fingre through it and the glaze not come back together. Voila!! let cool and pour over anything... Only takes a few minutes, the time it would take you to melt the icing in the microwave, not to mention taste so much better...