Rotate a transform's Quaternion by a Vector3 axis

Hello there. I'm working on a space exploration game and I'd like to add rotation axes for the planets. Each planet should have a rotational axis. In the initialisation of the planets, I'd like their transform.rotation Quaternions to be rotated to 'match' their rotation axis.

I've seen how you can .Set a Quaternion's rotation using a Vector3, but (unless I have this wrong) that method provides x, y and z as degrees of rotation; I'd like to rotate the Quaternion by a normalised Vector3 axis, which I can then use as the 'up' in the planet's on-going rotations.

I hope that makes sense. Any idea how I can rotate a transform so that 'up' on the model matches an axis?