United MileagePlus (Consolidated) - Adding segment after ticketed on MP Award ticket?

Hey FT friends,
I am turning the 4-0 in September and want to celebrate. My goal is to get back to Cambodia and possibly Myanmar in August. And like most others on here, I want to sample a variety of great First Class cabins. My schedule is flexible that time of year so I can do close in tickets. Specifically I want to go via the Atlantic on LH in their new First Class Cabins through FRA and NRT to experience the 747-8, 380, and of course the FCT in Frankfurt. Although LH has a direct to BKK, it would be a crapshoot getting the old First Class, so I would rather not take that chance and instead go through NRT.

My ideal itinerary would be the following
LAX-FRA
FRA-NRT
NRT-BKK
from BKK I would find my own way into Cambodia.

My concern is the MPM issue, so I figure I might have to originate in ORD (assuming the 747-8 is in service on that route by August) or IAD to keep the mileage in check.

So here is the question. When I go to book, am I better off just starting with ORD/IAD and trying to add the segment from LAX to either point after the fact? Or better off trying to do it from the start. Not even sure if segments can be added once ticketed.

Any thoughts or suggestions?
Thanks!

I have been trying unsuccessfully with great flexibility on dates for a while on many routes going via FRA w no success yet.

I called yesterday to book a "sample" to see how it would go. The agent said F on the 380 was open and I could book it for the 11th of June, two days from now. My sense is that it can be done as long as it is very very close in.