I am a little uncertain about what you mean by "plain hex file"; that is, whether you mean a raw (binary) file, or a file that is still in an ASCII HEX format, but with the Intel header and trailer information stripped out.

If you want to convert Intel HEX to binary, there is a utility to do so on Keil's website here:

http://www.keil.com/download/utility.asp

If you want to convert Intel HEX to binary and then back to a file containing only ASCII HEX characters, you could easily adapt the ihex.c module that is part of the ATS5xISP programming utility on my 8052.com User Page here:

http://www.8052.com/users/dhenry