I think what Falko is trying to say is that there is no "modding" way of doing what you want with crafts.
As it is currently hard coded in the OpenXCom C++ codes, manufacturing can only consume items. Since a craft is not an item, manufacturing cannot consume a craft, no matter how you approach the problem within the current possibilities offered by the ruleset.
Of course, if you want to make your one version of the C++ code that allows crafts to be consumed in manufacturing, then it is possible. But that would be creating your own version/branch of OpenXCom, and would essentially not be OpenXCom anymore since it would use different C++ code.
There is a realm of the possible that encompasses pretty much everything we could dream of, a subset of that is the realm of everything possible to modders as currently supported by OpenXCom. Unfortunately, what you want to do with crafts is within the former but not the latter. Either make it happen using C++, going beyond what is possible for modders into what is possible to coders, or it is indeed impossible.
TL;DR: "I want to use the ruleset and make manufacture take a craft as a component" is impossible. "I want to go and change the OpenXCom C++ code to make manufacture take a craft as a component" is possible, but much more work and not really considered modding since it tampers with the executable instead of its ruleset.
I think your best alternative as a modder would be to increase the price by 600k, and the time by 72 hours (the ordering price and delivery time of an interceptor), to represent ordering a brand new interceptor and modifying it as soon as it arrives.