For something that has already been recorded, one could convert the Left/Right stereo to Mid/Side, and make a M/S ratio adjustment (bumping up the Side level and reducing the Mid) before converting back to L/R stereo again. However, that's dependent on having sufficient stereo difference (Side) information available in the original file. Many DAWs and wave editors have a "super-stereo" or "over-wide" option on the stereo panorama control.
A more advanced variation on the above is EQing the Mid and Side components separately before converting back to L/R. That sounds like what you are suggesting, Peter.
There are stereo adjustment plugins which make use of this kind of M/S ratio adjustment and do the above conversion for you. Likewise, many stereo EQ plugins now offer modes allowing direct adjustment of Mid and Side instead of Left and Right. Any of those can do both tricks. To use such an EQ mode for straight M/S ratio adjustment without any EQ change, set it to M/S mode and simply change the relative gains of each channel without specifying any EQ filtering.