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;
}