Have you already written your web service? Is everything working correctly in it?
In general, if you have a web service that can retrieve and submit the data, you should be able to add a data connection that retrieves the data (and put a table on your form to display it) and then set up another connection for submitting your data back to the correct web service for submittal.
If you haven't already written your own web service, or if you are struggling with it, may I suggest you try ours? We have a free trial, and a new, very low price 'beginner' license available that allows you to use the web service for submittal of one form, up to 1,000 xmls for only $395.