You should be able to put just about any common type of paint on top of the powder. I'm most familiar with automotive paints and they should work great. I would tape off..tightly (use a fine line tape) the areas you don't want to second coat and then use a scotch brite pad (Red = aggessive / Grey = medium) to dull out and rough up the surface of the powder coat where you'll put the second coat. Follow the instructions of the paint mfg for surface prep and application.
Adding a "paint" clear coat will smooth the tape lines and edges but... You'll have to coat the whole project. To get a good wet coat to flow and layout will probably lift the decals, maybe even lift and pucker them up because of the solvents used to flow the clear. If your coater is good with custom tape work maybe you can use powder for the accents.