Doesn't look like you can specify this using the API. You should submit an enhancement request to the API support team.
If you're really desperate then there are ways to manipulate the SolidWorks GUI using the Windows API, but this is very complex. My team has this kind of experience so shoot me an email if you need consulting.
Thanks a lot Keith. So I'm not blind. Because I use the the item numbers to manipulate our EDM-system I can read out the numbers and make my own increment.