You will need to create a custom function.  Here is working example.
public static string GetUserDescription(object UserID)
{
    string resolved = string.Empty;
    if (UserID != null)
    {
        int userID;
        if (Int32.TryParse(UserID.ToString(), out userID))
        {
            var user = UserInfoProvider.GetUserInfo(userID);
            if (user != null)
            {
                resolved = user.UserDescription ?? string.Empty;
            }
        }
    }
    return resolved;
}