Converting .dax files to .cso format.
Converting PSP Compressed ISO Disc Images directly to Compressed ISO Disc Images (.dax to .cso conversion) should be possible, even though your options might be limited.
Try the PSP ISO Compressor, which should allow you to easily convert .dax to .cso.
We are not aware of any online conversion solution
ISO Compressor is a free game ISO file compression software
A PSP ISO compressing tool