For instance:
“instrumentFamiliesDefinitions.xml”
“instrumentnames_en.xml”
“Instruments.xml”
If I make an additional .xml for each of these with new information, can I get Dorico to read them along with the originals?
No, Dorico only reads the files inside the application folder or bundle for these particular resources. You can of course modify the originals, but I don’t recommend it, as you may encounter unexpected problems when installing a later update (which might include the update failing to install, or your custom data being completely overwritten, and possibly other problems).
Is Dorico planning on adding user-defined instruments in the next release then?
I have a list of over 200 instruments to add.
I can’t say for sure whether it’s something we will add in the next version; we would like to add this functionality, but there are a lot of competing priorities. In general I can’t say that something will definitely be included until it has been implemented and passed through our testing processes.