If you need to add your own, new size parameters to a given standard in Inventor involute spline generator (shafts), you can do it through XML files.
E.g. you need to add a 1.75mm module to the DIN 5480 offering - specifically the size 62 mm - 1,75 x 34. For this change you will need to modify three XML files: DIN5480-30.xml, DIN5480-30_In.xml and DIN5480-30_Out.xml and add the missing size values. You can find these files in the Inventor's "Design Data" folder, specifically in:
C:\Users\Public\Documents\Autodesk\Inventor 2013\Design Data\Design Accelerator\Tables\SplinesInv\.
The modified versions of the XML files for this example (to compare with and replace the originals - backup them first) can be downloaded from CADstudio.cz - DIN5480.zip (21kB).