You can't - they're automatically generated. And as for scenery and people etc. etc. You can only change the main theme.
This is just from other questions I've seen. I don't even have GT5
In the course maker you do need luck to get good jumps.
However I got some with creating a course on Mt Aso (Tarmac), you actually mostly get some jumps in there.
I guess a simple high speed course the mountains will always give jumps after a few tries.
I created a course with 3 sections and reduced the complexity and turn sharpness down as far as I could. This has enabled my cars to get a good run at the jump where they take flight.