Is it possible to import XML into another XML file?

Hello everyone,

I am working with a really huge project and it's stored as a 6MB xml file. Could I make this XML in pieces in order to make it look more organised? Using something like this: https://www.w3.org/TR/REC-xml/#sec-external-ent

However, I tried to use entieties but Soap UI does not recognize it. I have about 150 test cases and I have on mind to do it like this:

Project folder/
      Main.xml
      inc/
           Header.xml
           Testcase1.xml
           Testcase2.xml
           Footer.xml

Would it be possible to have 1 xml for each test case and invoke them from Main.xml? 

As a result of this, when I upload it in Gitlab, the changes of developers will be pretty much visible. Which one is your approach in order to guarantee organistation in such a huge project?

Thanks in advance.