It's better all around not to replace the STR# by a new one with one more entry, but instead use the "handy string patching functionality" described in the Appendix III of the Bible. Though the IDs to use for this do not appear in the list (though an allusion is made to it in "For example, to change the first cargo type from food to something else, youÂ’d simply create a 'STR ' resource with the ID 9000 and type in the name of your new commodity"), they are 9000 and up to patch the uppercase cargo name (otherwise found in STR# 4000), 9100 and up to patch the lowercase cargo name (otherwise found in STR# 4001), and 9200 and up to patch the status display abbreviation of the cargo name (otherwise found in STR# 4002). Remember that the index of the cargo you're trying to patch (or add) is to be added to the base ID to obtain the ID to give to your STR, and that if you add a new mission cargo, you should of course avoid an already used index, or missions from Nova which use this cargo index will use your new cargo (which has no implication other than the name, but still it's odd to carry something else that what you've signed up to carry), so as the STR# ID 4000 contains 79 entries, this means (as the first entry has index 0, so the last has index 78) you have to make your STR resources with IDs of 9079, 9179 and 9279. And, of course, enter 79 in the cargoType field of your mission.
Attentive people may notice that there are 255 possible cargo indexes, which means the patching mechanism here is incomplete (only allowing patchin for the first 100 entries), probably why Matt Burch did not document this for mission cargo (only the 6 main cargo for the prices and status display).
For those that I wondering how I do know such an underdocumented thing, I just looked for it in the Bible, saw it wasn't there, but that something was fishy (hmm... there is an allusion to using STR ID 900, but it is not present in the list below...), and tested it. Yes, I'm currently trasporting Bastards instead of passengers in Polaris space.