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Hi BJ Mc,
SharePoint Online can only use BCS to connect to different data sources like WCF web services, O-Data sources and SQL Azure databases. By saying “We have created a WSDL to read the DB and it outputs the data to an XML file. How can we get that XML file into a SharePoint list”, do you mean the WCF has been configured? If so, please refer to the following article about connecting to WCF Service using BCS.
Manage Business Connectivity Service Applications
If not, you may try to see whether we can achieve this via writing some codes or PowerShell scripts. For this situation, we would suggest you post the question in our MSDN forum which is a specific support channel for development related queries. The engineers and members there might provide you with professional support about your requirement.
Thanks for your understanding.
Allan