How can I convert a .png file to .icns

You can use:

 png2icns file.icns file.png

The first one is the name you want your .icns file to have. The last one is the .png file you want to export as an icon.

Important: Only convert the icons that match sizes icns supports: (16x16, 32x32, 128x128, 256x256, 512x512 and 1024x1024 pixels)

Or you can use an online service: iConvert Icons

Source: Ubuntu manuals