You should create custom Field Template MultiLineEditor_Edit.ascx (copy from MultilineText_Edit.ascx Field Template) and then add to MultiLineEditor_Edit.ascx, for example, HTMLEditorExtender.
Then edit your metadata:
[UIHint("MultiLineEditor")]
public object ColumnName { get; set; }
For more information please read article How to: Customize ASP.NET Dynamic Data Default Field Templates.