No worries theonly_one, I got a bus up there legally ^^
Jump both a packer and a bus up to the roof you started your runup on, then line up the packer so that you can jump the gap between the buildings with the bus. Once there, blow up the packer and get a Zebra cab ;-D
Of course it wouldn't be possible on PS2 (which is often the standard of whether something is legal or not) due to the distances between those vehicles, but it's technically possible to get a bus up there without handling mods or whatever.