Best way to combine multiple OpenAPI specifications

ExLibris' products Alma and Primo have comprehensive REST APIs, and similar to many vendors they divide these into multiple API Specifications.   This is a problem for me because:

(a) I will need to run swagger-codegen multiple times

(b) I will need to handcraft the deserialization process.  Even there I run into problems because there could be multiple inline body classes.

(c) Or, I will need to combine the different APIs I need by hand into one OpenAPI specification.

Online, I found a blog post that suggested using the npm package speccy to combine them - https://blog.runscope.com/posts/how-to-merge-openapi-definition-files.

But I get some messages about deprecated packages.  Is there any capability in OpenAPI itself to combine specifications beyond referencing schemas?     Is there a more maintained tool that does the combining I am looking for?