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Not sure what you mean by 'the CAS program', but if you want to make a CAS item that is packaged as a sims3pack rather than a .package, you do that with The Sims Resource Workshop (TSRW), from The Sims Resource:
http://www.thesimsresource.com/ . (But bear in mind that if you're making something to upload here to MTS, CAS items are not accepted in sims3pack format here, they're only accepted in .package format.)