matthieu, 
well your new version of combine_vol_data_adios.F90 would run only with adios databases files at the moment. if a user chooses to compile with adios support, but doesn't select it as enabled in the Par_file, he would have to re-compile with a different configuration setup. 

it turns out that i am such a user whenever i try to bug fix things and compare between original and adios outputs. thus, it would make my life much easier having to configure&compile only once... so if possible, try to make your version run-time dependent and the replacement is fine with me :)