X83 file is a GAEB DA XML Data File. GAEB is a German organisation issuing a special file format, commonly used in Germany for data exchange in tendering and accounting.
You need suitable software like GAEB to open an X83 file. Without proper software you will receive a Windows message "How do you want to open this file?" or "Windows cannot open this file" or a similar Mac/iPhone/Android alert. If you cannot open your X83 file correctly, try to right-click or long-press the file. Then click Open with and choose an application. You can also display an X83 file directly in the browser. Just drag the file onto the browser window and drop it.
However, here is a link that can lead to information about the software that could convert your X83 files into Excel files which could then be imported into ConQuest: