The only spa to png conversion we know about, would be export of special bitmap format used in MikuMikuDance (.spa) to PNG graphics. To our knowledge this can be only done with MikuMikuDance itself, but it might very possible that .spa file is actually just renamed .bmp file. Which means that it might be worth checking out to just renam the suffix from spa to bmp and use any graphics tool for export to PNG format.
Updated: August 28, 2023