Creating star effect in Pixelmator
This is most certainly possible in Pixelmator by using a brush in the form of a star. You can easily create one yourself straight in Pixelmator or you search on the net for an image of a star. Make sure the area around the star is transparent then you can save the star as a png-file and drag and drop that file onto the brushes palette. Now with the brush tool you can click on each blurry christmas light and cover it with the star. Try different blending modes for the brush. It could be you could get better results then. 

It is also possible to create a bokeh effect in Pixelmator. The guys at http://pixelmatortutorials.net have had a tutorial about this some time ago. The bokeh effect is done by applying one of the standard filters in Pixelmator.