There is no need to change the file extension - OpenMPT
never cares about file extensions. Just like with any Windows application using the standard Windows file browsing dialog, you can type "*.*" into the file browser to show all files, or select "All Files (*.*)" from the file format dropdown menu.
If OpenMPT cannot determine the format of a file, the
raw sample import dialog will pop up, allowing you to choose how to interpret the raw binary data. You can even go one step further and click the triangle next to the sample load button to open the dropdown menu and choose "Import Raw Sample". This will open the raw sample import dialog even for files that OpenMPT would normally know how to read (e.g. WAV files).