RAMAsset.getAssetAttributes() and RAMAsset.getAssetAttribute(String) can be used to get the available attribute on an asset based on the asset type constraints. After fetching an attribute from an asset, use RAMAsset.setValues(String[]) to set the values of the attribute.
AssetAttribute attribute = newAsset.getAssetAttribute("Custom Attribute");
attribute.setValues(new String[]{"value 1", "value 2", "value 3"});
session.put(newAsset, new NullProgressMonitor());