For small XML files, you could use ExtractText to get the entire content into an attribute, then UpdateRecord with an XMLReader, adding a property for your new field (let's say "/content") and whatever writer you wish. You will have to specify the output schema for the writer, to include all the fields parsed by the XMLReader in addition to a CLOB/BLOB/String "content" field. If you want to exclude fields from the XML then just exclude them from the output schema. Then you can use a similar Reader (AvroReader if you use an AvroRecordSetWriter, e.g.) in PutDatabaseRecord.
If this doesn't work for your use case, you may need to use SplitXml and work on individual records. This will degrade the performance of PutDatabaseRecord unless you merge the records back together later (using MergeRecord for example).