I believe that its possible to manually set the shear value, but that would mean that you're 'fighting against' (overriding) the animation's shear key values, and so I don't think it would be recommended in most cases.

What is it you're trying to do? There might be an easier way to do it than to directly set the shear.

Please see the documentation pages on how to directly set the shear values at a Bone:
Bone shearX
It is recommended to use one of the callbacks of SkeletonAnimation to override the Bone values after the animation has been applied:
spine-unity Runtime Documentation: Life cycle
When modifying e.g. the Bone local shearX value you can register your method to SkeletonAnimation.UpdateLocal.

As Jamez0r mentioned, you might want to consider whether you are better off with e.g. an animation playing on a higher track, setting shear values in a more artistically controllable way.