Im not sure if you are asking if you can make the time to reach 100 kts shorter or if you can make the plane liftoff at a slower speed than 100kts so ill try to answer both.
1. To make the plane reach 100kts faster leave the emergency brake engaged and let the engines spool all the way up before releasing. Or you could modify the cfg or air file (not sure wich one) to make the engines more powerfull.
2. To make the plane liftoff at a slower speed than 100kts try loading less fuel and lowering your payload weight. Or you could modify the cfg or air file (not sure wich one) in some way to accomplish this.
Probably not much help as I don't tamper with my files too much. But Im sure someone will pop along shortly with a more accurate answer .
good luck