Hello,
There are several tools that interact with save game files.
f1grid - Allows the modification of grid positions within a saved game file.
setlevel - Allows the modification of the game difficulty level within a saved game file.
setnames - Allows the modification of driver and team names within a saved game file.
gplist - Tool that extracts data from the save game files.
I'm not aware of any tool that can modify the qualifying session length. I also don't think we have any documentation specifically related to this.
Possibly something could be reverse engineered using the available tools. I see that gplist comes with the source code, so that could help.
Cheers