In response to my own question (for the benefit of others) as I continued to explore this basic question:
Currently Tiki can only import "Tiki-databases". phpMyAdmin, however, can open and reveal the configuration of the Tiki database and what php files are related to particular tables, and facilitate the importing of data from data sources that are compatibly configured.
Such configuration of a db (Sql, CSV, etc. to be imported), if extensive (such as adding additional columns to tables and populating the records in those new columns with data using scripts, macros etc.), can be facilitated by tools like Excel and Access, for example, and the associated php files provide clues as to the data sources and how they are derived (for example, I see some forum table record content is hashed from other records, the resulting hashed record is then used in truncated form in another record, etc.).
Of course, taking Tiki "off-line" and backing up the existing Tiki Sql database before modifying it by port or import is more than just a "good idea"..
In my specific situation, I'm porting some Yahoo groups (of which I've captured using a program called "PG Offline" prior to Yahoo removing the groups from on-line support), which can export that data in CSV and Sql formats, for reconfiguration and ultimate import into Tiki forums.
Why? Yahoo is converting all their Yahoo groups to exclusively email distribution lists, which is unsatisfactory for the comprehensive functionality my groups require. Tiki provides that comprehensive functionality, and supports such email distribution lists in their forum capabilities (which is why I chose Tiki over many other forum options). This allows me to migrate the groups while seamlessly supporting those members who primarily use(d) email for interfacing with the Yahoo groups forum for messaging, and only went "on line" to access stored data (which is no longer available via Yahoo as of 14 Dec.).
Specifically, now that I've been able to mount both the source database and destination database it appears at first glance the task of making the PG Offline data output compatible with Tiki is "relatively simple" if you have database manipulation experience (mine is about 20 years old, but "it's coming back to me" as I get into it further).
Assuming successful completion of this effort I'll then report the actual simplicity (or lack thereof), either in this thread, or another if the "guru's" inform me there is a more appropriate repository.