Is there a way to modify bone’s axis after they’re created?
Do you mean armatures? Just enter edit mode with your armature selected and move them around any way you want.
If you mean: is there a way to shift a bones center, the answer is no. I think. There are some sliders in the edit buttons that MIGHT do it. I will check on it when I get a moment.
You can change the ‘roll’ of the armature, with the NKEY menu. This is useful for aligning the Z-axes upwards.