On the Project tab, in the Add group, click Add Files .
Locate and open the folder that contains the file you want to add.
Click Open.
Do one of the following:
Click No, to add plain text files to your project. The Topic Preview displays the content of the files. You can export the content of the files to the output formats but you can not edit the text in the Doc-O-Matic Topic Editor.
Click Yes, to convert files into DTX files. The content the files is saved as DTX files and added to your project. You can edit the text in the Doc-O-Matic Topic Editor. The files are not altered in any way.
To associate different filename extensions with XML files
On the Quick Access Toolbar, click Settings .
Click the General Settings tab.
Select [General Settings] > General > File Types.
In the File list, select XML Files.
Click Change.
In Extensions enter the filename extension you want to add.