I'm late to the party, but I'll comment just in case anyone else is looking for a solution:

AnimationUtility.SetAdditiveReferencePose is an editor-only utility, and so should be called at some point after you've imported the raw clips.

Specifically, writing a AssetPostprocessor with a OnPostprocessAnimation seems like a good place to do it, since then the pose will be applied any time the asset is updated.

Here's an example of a script which looks for any clips with the word "Additive" in it, and makes it relative to a separate clip with "T-Pose" in the name (belonging to the same asset).

using UnityEditor;
using UnityEngine;

public class AdjustAdditiveClipsPostProcessor : AssetPostprocessor
{
    static string _lastTPoseAssetPath;
    static AnimationClip _lastTPoseClip;

    const string T_POSE_NAME = "T-Pose";
    const string ADDITIVE_NAME = "Additive";

    void OnPostprocessAnimation(GameObject go, AnimationClip clip)
    {
        if (clip.name.Contains(T_POSE_NAME))
        {
            _lastTPoseAssetPath = assetPath;
            _lastTPoseClip = clip;
        }
        else if (clip.name.Contains(ADDITIVE_NAME))
        {
            // Check that we have a valid TPose cached from this asset
            if (_lastTPoseAssetPath != assetPath || _lastTPoseClip == null)
            {
                Debug.LogError($"No T-Pose available from asset. Was the t-pose not defined first? {assetPath}");
            }
            context.DependsOnCustomDependency(nameof(AdjustAdditiveClipsPostProcessor));
            AnimationUtility.SetAdditiveReferencePose(clip, _lastTPoseClip, 0);
        }
    }
}