https://tech.blog.aerie.jp/entry/powershell-advent-calendar-2018-11

CustomProperty.cs

using System;
using System.Management.Automation;

namespace MyTypeAdapter
{
    internal class CustomProperty :
        PSAdaptedProperty
    {
        public CustomProperty(
            string name,
            Type propertyType,
            object tag)
            : base(name, tag)
        {
            if (propertyType is null)
            {
                throw new ArgumentNullException(nameof(propertyType));
            }

            this.PropertyType = propertyType;
        }

        public CustomProperty(
            string name,
            object value,
            object tag)
            : base(name, tag)
        {
            if (value is null)
            {
                throw new ArgumentNullException(nameof(value));
            }

            this.Value = value;
            this.PropertyType = value.GetType();
        }

        public override bool IsGettable
        {
            get
            {
                return true;
            }
        }

        public override bool IsSettable
        {
            get
            {
                return true;
            }
        }

        public Type PropertyType { get; }

        public override string TypeNameOfValue
        {
            get
            {
                return this.PropertyType.FullName;
            }
        }

        public override object Value { get; set; }
    }
}

StringPropertyAdapter.cs

using System;
using System.Collections.ObjectModel;
using System.Linq;
using System.Management.Automation;

namespace MyTypeAdapter
{
    public class StringPropertyAdapter :
        PSPropertyAdapter
    {
        public override Collection<PSAdaptedProperty> GetProperties(
            object baseObject)
        {
            string strValue = LanguagePrimitives.ConvertTo<string>(baseObject);

            var properties = new Collection<PSAdaptedProperty>();

            for (int i = 0; i < Math.Min(strValue.Length, 3); ++i)
            {
                var property = new CustomProperty(i.ToString(), strValue[i], null);
                properties.Add(property);
            }

            return properties;
        }

        public override PSAdaptedProperty GetProperty(
            object baseObject,
            string propertyName)
        {
            var properties = this.GetProperties(baseObject);
            var property = properties.SingleOrDefault(p => p.Name == propertyName);

            return property;
        }

        public override string GetPropertyTypeName(
            PSAdaptedProperty adaptedProperty)
        {
            if (adaptedProperty is CustomProperty cp)
            {
                return cp.TypeNameOfValue;
            }

            throw new NotSupportedException();
        }

        public override bool IsGettable(
            PSAdaptedProperty adaptedProperty)
        {
            if (adaptedProperty is CustomProperty cp)
            {
                return cp.IsGettable;
            }

            throw new NotSupportedException();
        }

        public override bool IsSettable(
            PSAdaptedProperty adaptedProperty)
        {
            if (adaptedProperty is CustomProperty cp)
            {
                return cp.IsSettable;
            }

            throw new NotSupportedException();
        }
        
        public override object GetPropertyValue(
            PSAdaptedProperty adaptedProperty)
        {
            if (adaptedProperty is CustomProperty cp)
            {
                return cp.Value;
            }

            throw new NotSupportedException();
        }

        public override void SetPropertyValue(
            PSAdaptedProperty adaptedProperty,
            object value)
        {
            if (adaptedProperty is CustomProperty cp)
            {
                cp.Value = value;
            }

            throw new NotSupportedException();
        }
    }
}