Christian,To put a repeat bar in the middle of measure, you actually have to create two measures. In your example, do the following:
Measure 9 and 18 need to become 2 2/4 measures each but displaying 4/4 as the time signature.
Place the repeat mark where you want it in both measure and make the barlines invisible where the repeat bar is.
If you are using meausre numbers, you will have to create new measure regions to account for the 2 *extra* bars.
For more detailed info, check the manual section on using a different time signature for display (page 12-4) and measure number regions (page 14-25). BTW, the page numbers are the ones for Finale 2002, Mac. Your page numbers may vary depending on finale version and platform.
Jim Coull