carcassonne.de offers a special download
here which is an extra scoring table, covering 50-300 points. You could stick that on a piece of card, and then only hand out the normal scoring tiles when you pass the 300 mark. Also, you'd need an extra meeple or marker for the new score-board.
Other than that, the only way I know of getting more scoring tiles is to buy the basic game again…
A score of 600 using all expansions is high, but not surprising. But what do you mean by 'frequently have to place men on the scoring tiles'? There should only be one guy on the score-board; when he finishes a complete circuit of the board, he (she/it) goes back to zero and you take an extra scoring tile. Where do the other 'men' come in?