While it might be possible for special cases [1], I have my concerns, you really can't compress (lossless) beyond the (information theoretic) entropy -- in which case one would try to compress deflate (zip) with deflate (png).

[1]: I remember when I was taking an information theory lesson at the university and told the (video/jpeg compression guru) professor that I compressed an AVI file with my dump arithmetic coding implementation, he was shocked, turns out the file had some large crappy header from the editor

> another few % of compression

ZIP consists of sections of both compressed and uncompressed data, so some compressibility is to be expected.