My name is Jack Kersbergen. I have the complete Kersbergen genealogy you seek. I grew up in Rock Valley Iowa and your grandfather William was my uncle. His father, your great grandfather, was John Kersbergen and his mother was Martha Velgersdyk Kersbergen. Your great grandfather was a member of the US Cavalry and fought in the Spanish American War with Teddy Roosevelt and also the Chinese "Boxers Rebellion" (see movie The Sand Pebbles w Steve McQueen). He eventually returned to Rock Valley where he was the town backsmith.
The genealogy i have goes back to the origin of the Kersbergen name in Utrecht, The Netherlands, where there was a Kersbergen estate "de Ridder Hofstad Kersbergen" . ther is also a Kersbergen Coat of Arms with a shield and 3 red sickles that is recorded in Dutch books.
The genealogy was developed by Pat Oosten of New Port Ritchie, Florida who is married to Roger Oosten who is my cousin. Roger is the son of Henry Oosten and Martha Kersbergen Oosten originally of Rock Valley Iowa also and Martha was a sister of your grandfather William.
I can e-mail the genealogy file in Ms Word to you.