In general, if Kellogg decides you're good enough for them, the law school will accept you too. The application process reflects this.
In fact, it's actually incredibly easy (based on anecdotal evidence) to go from just the MBA program to the JDMBA program after you've been accepted to Kellogg. You basically just ask for a spot. This may vary from year to year, (and in fact I can't guarantee it's still the case) but at least one 3J did this.
It does not work the other way around; if you're a JD and want to do an MBA, you have to do the full application.