FilipVolvo,
since the linkage section is meant as an area for two programs to communicate with eachother, i fail to see the significance of defining constants in it. As Crox indicated, it is impossible, too. However: the calling program provides a reference to parts of its working storage in the USING ... clause, so of course you can provide constants there, if you want. These constant values then are simply information passed to the called program.
Hope i was able to contribute something.
Good luck !