Yes, it is possible to change the qualifying times of other drivers.
You need to change the BLAP value in the <track>.TXT file. This sets
the absolute best lap other drivers can achieve at the track. The qual
times are calculated using this time, the RELS value, and the driver strength
setting.
You can also change the RELS value to adjust the relative strength of the
drivers in both qual and race. Since I'm still learning to drive road courses
well, I adjust down the RELS value for road tracks, and up for short ovals. This
lets me drive a championship at 100% opp strength and get a consistent challenge
throughout. BTW, the Michigan RELS value starts at 96, which may explain all the
complaints about slow opps at Mich.