The file you try to import into StageLight should be an .sfz file. The final issue might be that the location of the sample files on your device might not match up with what’s specified in the .sfz file. For testing purposes, here's a
zip file which contains a string folder having two .sfz files and two folders containing the samples which have been named to match the .sfz files. If you put all of the unzipped files and folders in the appropriate location for your device, you should be able to import and use them successfully. The credits for the two sfz files are listed inside of each. I was able to successfully import these two sfz files into the StageLight iOS app.
Tiny Piano