While I don’t know for sure the settings in extconfig.conf can be extended this way, most of the rest of the “entries in square brackets” (like contexts) can be “added to” using the “+” syntax. So, for example, if I wanted to extend the functionality of a FreePBX managed and standard stanza (even though this example is completely made up) called “DaveData”, in the immutable (FreePBX written) file, the context “[DaveData]” would appear. If I want to “extend” that context, I could add in (pretty much any) file that gets loaded later a custom context called “[DaveData]+”. This tells the system that I want to add whatever is listed below (in this contextual reference) to the existing context called “DaveData”.
This technique is often used to extend the functionality of things like PJ-SIP or the special contexts added as part of the support for “Chan-SCCP-B” channel driver. This way, the system’s standard contexts aren’t overridden, it’s more like they are overloaded (in the C++/object oriented meaning) to extend the context to include functionality that the standard stanza doesn’t normally include.
Note that this extensibility is available for many parts of the system, but I don’t know if extconfig.conf is one such system part. One attempt recently to do this for PJ-SIP failed because the context name was reused several times for different things and the system wasn’t sure which of these duplicates needed amended. My suggestion would be to add it to one of the *_custom.conf files that the system already supports and see if it works. If it does, you’re golden: if it doesn’t, you know one solution that doesn’t work and are no worse off than you are right now.