Wisdom wanted -- how to inject tele-extender info

[garyaitken]

I'm using some legacy lenses (leica) with tele-extenders on them on an olympus e-m5 body.
There are standard tags for the lens information (LensInfo, LensMake, LensModel, LensSerialNumber),
so injecting that is no problem.  There are even leica / panasonic text strings for sorta-similar lenses to allow me to get the text strings sorta-right.
However, I don't see any tele-extender info in the standard exif data; maybe I missed it.
I do see olympus-proprietary info about tele-extenders:
---- MakerNotes:Olympus ----
Extender                        : None
Extender Serial Number          :
Extender Model                  :
Extender Firmware Version       : 0

Is there a recommended way to add this information?
I know to adjust the exif values FocalLength and FocalLengthIn35mmFormat to account for the extender, but if it makes sense I'd like to include the fact that there was an extender on, and what the make, model, and serial number were.  I'm leaning towards using the olympus proprietary fields ExtenderModel and ExtenderSerialNumber, but am wondering if there is a better alternative.  I don't know that it's wise to use the Extender field as that is encoded to represent olympus-proprietary products.

Any and all suggestions welcome.
Thanks.

[Phil Harvey]

The Olympus fields are strings, so they would work.  If you wanted to store it in standard EXIF, I think you'd have to put it in with the lens name.  An alternative is to create a custom XMP tag and put it in there.  The XMP solution is maybe the best one for forward compatibility while keeping the extender information separate from the lens name.

- Phil

[garyaitken]

Thanks.
Point of clarification for others reading:
The plain "Extender" field is not a string, it's an array of 6 int8s, at least according to the posted info:
  https://exiftool.org/TagNames/Olympus.html
Other fields are text as noted above.

[Phil Harvey]

Right.

- Phil