Solved this!

Using this method to generate string for csv import file:

private static string ToMetaStringDictionary(IList<string> list)
        {
            var dictionary = new MetaStringDictionary();
            var serializer = new JavaScriptSerializer();

            for (var i = 0; i < list.Count; i++)
            {
                dictionary.Add("Bullet" + i, list[i]);
            }

            return serializer.Serialize(dictionary);
        }

It will generate something like this:

"[{""Key"":""bullet2"",""Value"":""value 1""},{""Key"":""bullet1"",""Value"":""value 2""},{""Key"":""bullet0"",""Value"":""value 3""}]"

and use this in mapping file:

<RuleItem>
    <SourColumnName>Bullet Point List</SourColumnName>
    <SourColumnType>System.String</SourColumnType>
    <DestColumnName>BulletPointList</DestColumnName>
    <DestColumnType>StringDictionary</DestColumnType>
    <FillType>CopyValue</FillType>
    <CustomValue />
    <DestColumnSystem>False</DestColumnSystem>
  </RuleItem>