You probably need to make the project recognize the file's extension. Proceed as follows:
1) Go to the Project's Settings panel (press ALT-F7)
2) Click on the File Mappings selection at the left. The File Mappings panel appears.
3) Click on the Choose button, and navigate to your file. Click Open.
4) Information on the file should appear in the controls. For the Compiler setting you probably want to pick None.
5) You may want to experiment with the Flags menu. If it's the be incorporated into the final module, you'll want to leave Ignored by Make unchecked.
HTH.
---Tom