Even though your question does lack a lot of important information (e.g. what HANA version you're on and whether or not you are using XSC(lassic) or XSA(dvanced)) the answer is: yes, you can access and use tables outside the CDS managed schema.
The way to do that is to make the tables available via Synonyms and reference those synonyms via the "using" command in your cds file.
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