As things stand, modding quests is possible and a few have done it. But its rather telling that the ability to do so has been around for
years and there aren't many: making even linear quest chains is quite difficult, needs a lot of writing and coding, and requires understanding several different, mostly undocumented systems. The results are also more difficult to copy, because there is no one "right" way to make a mission: I expect that every current mission that a modder could look at as an example uses different methods and structures.
Wisp has been making a huge effort to make things more manageable, and is writing a guide here:
https://starsector.fandom.com/wiki/Modding_QuestsIt isn't complete yet, but it outlines most steps of what is needed and gives crucial documentation.
Wisp has also been working on a library to make things easier, and I believe at this point it is relatively complete. Its written in Kotlin, but I believe they are working on a Java port to make it more accessible.
I am hoping that next SS version has many more missions to go along with the introduction of story points, and that things will be cleaned up under the hood and made more accessible.