There's no way to do that -- Zotero has a single call number field. I see why this would be occasionally useful to have multiple, but overall I don't think there's sufficient demand/use cases for that level of additional complexity.

The question is, "What is the prevalence of libraries throughout the world that use Dewey vs. LOC vs. some other system?". Another question would be, "Why have a generic call number field when there are several common cataloging systems each of which is exclusively used in a particular library?". Does CSL and Zotero need to consider including system-specific call number fields instead of a single-generic field?

For example, over the years in my (non-Zotero) database, to facilitate "interlibrary loan" exchange processes, I've needed to include multiple journal identifiers. ISSNs "should" be sufficient in most cases but some USA libraries insist on using the OCLC number while many hospital-based medical libraries prefer the US National Library of Medicine serial number instead of the others. It seems that the cost of providing a copy of an article can vary by the type of identifier used by the receiving and requesting libraries. I still receive requests for the inclusion of other numbering systems used in Europe and Asia (but these systems have not assigned entity numbers to many thousands of existing journals). Simply because we lack the funds to do it right now doesn't mean that we will not include several cataloging systems' call numbers for books in the future.

edits to fix spelling