If any kind of spa to jpg conversion is possible, it would be export or conversion between two raster formats, from MikuMikuDance pictures (.spa) to JPEG format (.jpg). Such export of what probably are textures, can be done using the MikuMikuDance software only.
But SPA format seems to be just renamed BMP format, so if you rename the suffix from spa to bmp, you can most likely use any bmp to jpg conversion solution for this as well.