I don't think this is a good idea for several reasons.
Firstly, loading this kind of crypto related code from some CDN I never heard of seems like a very bad idea. It may give a false sense of security.
Secondly, libsignal is licensed under GPL. The same reason it's not included in the Converse.js bundle may be a valid reason to not include it here.
In short, if you want OMEMO, I recommend you do this change locally.