How can I import an XML file from an external source?

Depending on the nature of the data, you might be able to pull out some unique values, and use those as the key fields and the relationships.  Then you can just import the individual sheets to the appropriate tables, and boom, you've got it.

But I really have no idea about your situation.  Do you have more details about how the data is all connected and how you are hoping to use it in QB?

MLS XML feeds can be highly complex data structures and often are in proprietary formats or have access and usage restrictions. Even OpenMLS is far more complex than a CSV file or the XML structures commonly seen within QuickBase or Excel.

Here is a old XML file  in OpenMLS format:

http://www.xml.com/pub/a/98/08/real/xmlexample.html

https://pastebin.com/raw/hfLLdTks

Be that the case, you apparently have access to a MLS XML feed and want to get it into QuickBase.

Similar to your other question, you would have to write a script to take a file selected through a miniature form (ie <input type=file>) and create a FileReader object. Once the FileReader loaded you would have to parse the XML file for the pieces of information you want to import and build CSV strings. Depending on your needs the CSV would be imported into one or more tables. The exact details would depend heavily on the structure of your tables as well as the structure of the particular MLS XML feed you are using.

Today leading developers use JSON and normalize various MLS XML format to a common JSON represenation.

If you need further assistance with the issue please feel free to contact me off-world using the information in my profile:

https://getsatisfaction.com/people/dandiebolt