Can I do this with additional segments?
 

I have a RTW F ticket (ex-BKK). If I want to add segements for North America, can I add a Business Class segment for $400 and but then I can travel in First on a 2-class plane. That would be cheaper that purchasing a First Class segment for $550.

Can they issue a 'D' segment on a 2-class service?

Absolutely Except in certain transcontinental routes which have three class service . I also buy an economy segment and upgrade using 5000 miles

BA Gold:

Here are the exact rules as taken directly from Sabre:
2 ADDITIONAL FLT SEGMENTS WITHIN EACH
CONTINENT - EXCEPT CONTINENT OF ORIGIN-
MAY BE PURCHASED AT A CHARGE PER
SEGMENT OF:
FIRST - USD550/AUD750/GBP350/
HKD4300/EUR450.
BUSINESS - USD400/AUD500/GBP200
HKD3200/EUR300.
ECONOMY - USD150/AUD200/GBP75/
HKD1200/EUR100.
FLT SEGS BEYOND THE NUMBER PERMITTED
AT THE THROUGH FARE MAY BE PURCHASED
IN THE SAME OR LOWER CLASS OF SERVICE
THAN THAT OF THE OW EXPLORER.

Since you have an F class OWE...you can buy either F, C or Y as the additional segments.

I'm not sure if the subtleties of the question have been answered. Its one I've never thought of and don't know the answer to.

On a 2 class AA flight there is A and M inventory but is there D inventory - I don't think there is. So if you buy an extra segment it will be in A and at the 1st Class supplement price. If the flight is a transcon there will likely be 3 class flights with A ,D and M inventory. You should be able to buy a D class segment and then if you change to a 2 class flight get upgraded to A class. But if its not a transcon route with this option, I would think you could only buy in A or M at the First Class or Economy Class supplement respectively.

This is just my reasoned opinion no more!

i would agree with quebec.

although the problem with booking a D segment is difficult because i believe you already used up your transcont sector.

(opinion only, again)
however, i believe you can specify your 7th segment. i.e. if you have 'flown' the transcont. you can re-issue your flight (with your new sector in A, and teh transcont in D). and you speficy the 7th segment as teh D sector.
later, (afetr re-issuance), you can rebook into a 2 class flight.

as otehrs have said, AA is notorious at re-booking RTW. so better have your re-issuance (and PNR) with CX/BA, adn hence also your later rebooking