Can I use the API to add hyperlinks in the document to areas within that same document? That is, can the API replicate/replace the functionlaity of the Link tool?
I was under the impression that Datalogics, ActivePDF, and Foxit all had SDKs which would provide that capability. Is that not correct?
You would need to ask ActivePDF and Foxit about their APIs. Datalogics license the Acrobat Library from Adobe, so it is on topic here. This offers a C++ library (and some other language ports). This can be used, by users who have some knowledge of internal PDF concepts and structures (not too much for this task), to create what in PDF terms is a "Go To Link" from an area on one page to an area on another page, specifying the zoom option. Reading around PDAction and PDAnnot in the API would be a start though you'd also need to know about PDDoc and PDPage objects, among others. (These are API abstractions, not names in the PDF specification).