Basically you are saying you want to create an XML file like
My understanding is that the answer is you cannot.
You can only get the attributes under the Project tag. The other tags like Resources, Tasks and OBS create hierarchy.
Some of the other options for creating the xml file are
Create a query based portlet with all the data you need and export to Excel
Use MS Word Mailmerge to create the xml file from the exported Excel file. That has to be saved first as a proper xls file first.
User Excel formulas to add xml tags to the data in your exported Excel file. First for jut one row of data and then copy for the subsequent rows.
Do it completely manually
Read out from the old system, edit the output files manually and write into the new system.
Use xml tools. There the learning threshold and setup may be the problem.
Mail merges could be the fastest way
Excel formulas give better options for formatting, but may be more cumbersome. Once you get going easy to make modifications. I have used this to create hierarchical OBS structures in Niku 6, 7, 8 and later.