I've discovered a way to do this using CoSMOS (all hail CH and CoSMOS!!!). Let me know if this works for you, or if you need more assistance. I've attached a screenshot of my CoSMOS screen to help.
1. Start the challenge w/the timer and pause the game, remembering how much time remained on the timer.
2. Start up CoSMOS in Skilled mode, select the BAC process, and start a Memory Scanner.
3. Change the value to float.
4. Scan for the exact number of seconds left on the timer.
5. Go back to the game and let the timer count down a couple of seconds.
6. Change the search type to Decreased Value.
7. Repeat step 5 and 6 until you have one memory address that matches the correct number of seconds on the timer. For example. I had 1:04 left on the timer, and the only value left that matched in my memory scan was 64.85592651. Ignore the numbers to the right of the decimal; the 64 matches the 64 seconds (1:04) on the timer.
8. Double-click the memory address in the top window (Scan Results) so that it shows up in the bottom window (Stored Items).
9. Change the value to 2400.
10. Return to the game. You should now have 40:00 on the timer.
You can re-modify the time throughout the challenge.