Unfortunately, in the version you are running, you will need to blow away your existing ACL and add it back (with the new deny rule before the more general permit).
It sounds like you should seriously consider upgrading to the latest 6.3 image. There you can use the 'line numbering' feature in the ACLs whereby you can add a new ACE in anywhere you want in the ACL just by specifying the line number.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...