Premiere Pro - Change indeterminate Media Time-base to custom value (say 12 fps)
When you create a new project, on the 'Settings' page, select 'Custom' from the 'Editing Mode' drop-down, then you can select '12fps' as the 'Intermediate Media Timebase'.

If you already have a project, you can create a new sequence and select 'Custom' and '12fps' as above in the 'Sequence Settings' menu.