The only viable png to fig conversions we know about thus far, is export of graphics files between standard PNG a TransFIG (.fig) files, something that cannot be done with regular graphics editors, simply due to luck of support of FIG format.
Your best option in this case is to use one of the dedicated graphics converters, as these usually support broad range of even legacy formats.
Another conversion, or more of reuse of PNG images, is with Figma interface design tool which supports import of .png files and uses .fig files to store projects. But that is not file conversion per se, more of how you work with the software.
Updated: October 2, 2023