Property "types" are handled by SlimeProperty instances. Whilst creating SlimeProperty objects is not allowed, there is a list of all available properties. Properties and their values are stored in SlimePropertyMaps.
Example Usage:
// create a new and empty property map
SlimePropertyMap properties = new SlimePropertyMap();
properties.setString(SlimeProperties.DIFFICULTY, "normal");
properties.setInt(SlimeProperties.SPAWN_X, 123);
properties.setInt(SlimeProperties.SPAWN_Y, 112);
properties.setInt(SlimeProperties.SPAWN_Z, 170);
// add as many as you would like