NavMeshMerge.cs

    using System.Collections.Generic;
    using UnityEngine;
    using UnityEngine.AI;

public class NavMeshMerge : MonoBehaviour
{
    public NavMeshMerge Instance;

    public MeshFilter meshFilter;
    
        private void Start()
        {
            Instance = this;
            Add();
        }

    public NavMeshSurface mainSurface;
    public NavMeshSurface[] surfaces;

    public void Add()
    {
        var data = NavMesh.CalculateTriangulation();
        
        var mesh = new Mesh();
        mesh.vertices = data.vertices;
        mesh.triangles = data.indices;
        
        mesh.RecalculateNormals();
        mesh.RecalculateBounds();
        
        meshFilter.mesh = mesh;

        foreach (var surface in surfaces)
        {
            surface.enabled = false;
        }
        
        //create new
        
        var source = new NavMeshBuildSource();
        source.transform = meshFilter.transform.localToWorldMatrix;
        source.shape = NavMeshBuildSourceShape.Mesh;
        source.sourceObject = meshFilter.mesh;
        source.area = 0;

        var sources = new List<NavMeshBuildSource>();
        sources.Add(source);

        var settings = NavMesh.GetSettingsByIndex(0);
        settings.agentRadius = 0.01f;
        
        var epochStart = new System.DateTime(1970, 1, 1, 0, 0, 0,System.DateTimeKind.Utc);
        var timestamp = (System.DateTime.UtcNow - epochStart).TotalMilliseconds;
        
        Debug.Log("start");
        var newData = NavMeshBuilder.BuildNavMeshData(settings, sources, mesh.bounds, meshFilter.transform.position, meshFilter.transform.rotation);
        
        NavMesh.AddNavMeshData(newData);
        
        var timestampEnd = (System.DateTime.UtcNow - epochStart).TotalMilliseconds;
        Debug.Log("end " + (timestampEnd - timestamp));
    }
            
}