Yes, polymer lowers can be painted. There are a few methods that can be used to do this, but the most common is to use a good quality 1-2 mil primer designed for plastic surfaces followed by a good quality automotive paint.
You will want to make sure that you thoroughly clean the surface of the lower before painting it. This includes removing any oils or other contaminants that may be on the surface. Once the lower is clean, you can sand it lightly with 1500-2000 grit sandpaper to create a rougher surface for the primer and paint to adhere to.
After priming and painting, you will want to sand again with 2500-3000 grit sandpaper and then apply a clear coat to protect the paint job.