where do you get the xml string from? I assume you are receiving it either from a file, in which case you are most likely using a file io adapter, or from a database, in which case you are using a db adapter.
The easiest approach, when already using an adapter that is not the xml adapter, is to parse the data using java in a custom step. Xerces is a good free opensource java library, which you can use for this purpose. Just be sure to include the xerces.jar file in the classpath when configuring the adapter, whichever type you are using.
One detail to pay attention to, if you ever find yourself in the situation where the EI tool is running on a different machine compared to the adapter, is to configure the Preferences | Session Data | external classpath (or something like it) with the path to the xerces.jar file on the machine running EI. This is required in order for EI to successfully compile the integration component with the parsing code in it.