Aah like that. If I think I know what you mean, you want to modify the collision data of each tile (where the player character will collide on a tile). It's a bit more tricky, but not impossible I think. In the graphics editor export the L1MS file, at 320 width, 672 height, 1 bitplane. This image contains the collision data for each tile, you will need to make the parts where the character will collide on grey, the parts where he will not collide on black. Then re-import.
I have not tested this though, so it may or may not work.