Mocking received XML sent to service

We have a situation where a 3rd party will be sending an XML file through that will talk to our web service and the service will then read the information and query the database acting on the information it receives (hope that makes sense!)

Can you tell me if I am able, if I have some dummy XML which has the same nodes that will be contained in the XML document the 3rd party will send, whether I can use SOAPUI to mimic this call?

If so could you explain how I would set this up.