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Is it possible too have a block, when on it you're able jump higher then you normally would when on the ground?

No
Or at least I don't know how....
Although you could make the ground a negative linear physics so when you are on a box it seems higher.....

Mahem1 is correct.

You would set _jumpVelocity to the highest value you could jump from, then add negative physics on objects to bring that value down to the amounts you would like.