You can use a TypeConverter that supports string conversion for this. For example, here is a TypeConverter for a property of Encoding type, that would be declared like this:

public class MyClass
{
    ...
    [TypeConverter(typeof(EncodingConverter))]
    public Encoding MyEncoding { get; set; }
    ...
}

/// <summary>
/// Provides a type converter to convert Encoding objects to and from various other representations.
/// </summary>
public class EncodingConverter: TypeConverter
{
    /// <summary>
    /// Returns whether this converter can convert an object of the given type to the type of this converter, using the specified context.
    /// </summary>
    /// <param name="context">An <see cref="T:System.ComponentModel.ITypeDescriptorContext"/> that provides a format context.</param>
    /// <param name="sourceType">A <see cref="T:System.Type"/> that represents the type you want to convert from.</param>
    /// <returns>
    /// true if this converter can perform the conversion; otherwise, false.
    /// </returns>
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return ((typeof(string) == sourceType) || base.CanConvertFrom(context, sourceType));
    }

    /// <summary>
    /// Returns whether this converter can convert the object to the specified type, using the specified context.
    /// </summary>
    /// <param name="context">An <see cref="T:System.ComponentModel.ITypeDescriptorContext"/> that provides a format context.</param>
    /// <param name="destinationType">A <see cref="T:System.Type"/> that represents the type you want to convert to.</param>
    /// <returns>
    /// true if this converter can perform the conversion; otherwise, false.
    /// </returns>
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return ((typeof(string) == destinationType) || base.CanConvertTo(context, destinationType));
    }

    /// <summary>
    /// Converts the given object to the type of this converter, using the specified context and culture information.
    /// </summary>
    /// <param name="context">An <see cref="T:System.ComponentModel.ITypeDescriptorContext"/> that provides a format context.</param>
    /// <param name="culture">The <see cref="T:System.Globalization.CultureInfo"/> to use as the current culture.</param>
    /// <param name="value">The <see cref="T:System.Object"/> to convert.</param>
    /// <returns>
    /// An <see cref="T:System.Object"/> that represents the converted value.
    /// </returns>
    /// <exception cref="T:System.NotSupportedException">
    /// The conversion cannot be performed.
    /// </exception>
    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (!(value is string))
            return base.ConvertFrom(context, culture, value);

        string name = (string)value;
        if (name == null)
            return null;

        return Encoding.GetEncoding(name);
    }

    /// <summary>
    /// Converts the given value object to the specified type, using the specified context and culture information.
    /// </summary>
    /// <param name="context">An <see cref="T:System.ComponentModel.ITypeDescriptorContext"/> that provides a format context.</param>
    /// <param name="culture">A <see cref="T:System.Globalization.CultureInfo"/>. If null is passed, the current culture is assumed.</param>
    /// <param name="value">The <see cref="T:System.Object"/> to convert.</param>
    /// <param name="destinationType">The <see cref="T:System.Type"/> to convert the <paramref name="value"/> parameter to.</param>
    /// <returns>
    /// An <see cref="T:System.Object"/> that represents the converted value.
    /// </returns>
    /// <exception cref="T:System.ArgumentNullException">
    /// The <paramref name="destinationType"/> parameter is null.
    /// </exception>
    /// <exception cref="T:System.NotSupportedException">
    /// The conversion cannot be performed.
    /// </exception>
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if ((typeof(string) == destinationType) && (value is Encoding))
        {
            Encoding encoding = (Encoding)value;
            return encoding.HeaderName;
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }
}