Use the CCP4 program "mama2ccp4" or:
"I have a utility written by Doug Ohlendorf which will convert a CCP4 mask to O/Rave format and vice versa. It requires FORTRAN and the CCP4 libraries, is guaranteed to run on SGI, and should be OK on most types of systems. the utility is called transmask. you can obtain it in this manner:
ftp indigo4.biomol.uci.edu login: anonymous password: your_bankcard_PIN_here ftp> binary ftp> cd pub ftp> get ncs.tar.Z ftp> quit decompress ncs.tar.Z tar xvf ncs.tar
(Dave Schuller on the CCP4 mailing list)