I've asked the same question before, and got the answer from Stewer, if I remember correctly. These are not his exact words, but the meaning...

Poser uses it's own internal format for the render cache, and there is no way to convert them into any recognisable image format.

Poser itself obviously has some internal conversion utility, and I dare say some clever Python person could find a way to harness it.

(Poser Pro can use an HDR-compatible format for its render cache, but this is no good to you).

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)