Use RAMAsset.getRatings() to get the ratings for an asset. Use rateAsset(int, String, boolean) to add tags to an asset.
RAMAssetRatings ramRatings = ((RAMAssetRatings)asset.getRatings());
ramRatings.rateAsset(4, "My First Rating", false);
session.put(newAsset, new NullProgressMonitor());