We have rolled the decks with Imron and non-skid additive many times and it comes out very nice. My brother has become an expert at it. As usual with paint, you can go as crazy as you want to on prep work. He's a fanatic so he goes at it pretty hard. But basically just sand/scuff, tape the edges, wipe down with a good prepping solvent, mix the non-skid into the paint and roll away. We usually do the cockpit and gunnels at least every 2 seasons to keep it fresh looking.
As Jim said, the key is to have someone keep stirring the mixture so that the additive does not settle. Oh, and I think my bro did use a flattening additive recently and liked it. Be sure to pick the correct roller for the job. One year we had a reddish roller bleed and turn the decks pink. Prepped and painted twice that year.
Also, be mindful of dew point, humidity, and the use of your air conditioning during the drying period. Seems that condensate can really screw up your efforts and those staterooms under the bow deck can get pretty cool compared to the outside temps.