The simplest way is to have the external H.323 endpoints call your CMS to join the meeting.
If you want your endpoints, or the CMS to be able to call out to the H.323 endpoints, you will need to do some tweaks on your Expressways to get them to interwork the calls from SIP to H.323 and send outbount - but the address you dial from internally will need to be a SIP URI (as your endpoints are SIP endpoints).
Some of this will depend on what the H.323 addresses you need to dial to are. If they're in a standard H.323 Annex O format (which very much resembles a SIP URI), or if you are able to massage them so they look that way (ie endpoint@ipaddress), then it'll make things a lot easier.
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