There's probably another stop on the return from Lewa to Wilson.
Discuss with your outfitter... they should know your international flight schedule in order to coordinate with your domestic flights.
If you decide to come in a day earlier, it would still be the same flight arrival time at Wilson.
How many days total you are yspending at Lewa? Can you/do you want to shorten stay at Lewa by 1/day? If so, then you'd need an overnight somewhere in Nairobi.
Final option, would be a private charter from Lewa into Wilson (or maybe direct to JKIA). This will be more costly, but compare price for private chrter vs domestic, the day at Lewa vs day in Nairobi, etc. Again, discuss with your ground operator in Kenya.