The Problem:
if you play offline, without using GOG Galaxy, you can't buy any quicksilver item except the ones introduced with patch 1.58. So you can't do the "Living Ship"-Questline, as well as many other cool things.
You also won't get any daily quests.

Solution:
Follow this tutorial to enable all Quicksilver items and also daily quests:
https://www.youtube.com/watch?v=JhbQaX9f8AM&feature=youtu.be

Original Post:
I play offline, without using GOG Galaxy.
I can't buy the new colored jetpack trails at the quicksilver trader, neither i can't buy the void egg for the new starbirth quest.

These are examples of what Hello Games calls "Community Events" where they track progress, have milestones towards completion, etc. They even have a "Galactic Atlas" webpage where one can see progress towards certain goals and milestones without launching the game.

I am not defending this but the bottom line is that as long as these are treated as community events they cannot be available without online access. Someone posted some detailed instructions on how one could temporarily go online to obtain these missions, rewards and items but that depends upon just how important getting those missions, items and rewards are to them.

There is no indication that Hello Games reads posts in this forum. They did make a post back in August or September 2016 and possibly one or two more but unlike the Steam forums and perhaps the console forums they do not even post information about patches and updates for GOG users indicating that perhaps they do not read these forums. Accordingly, a support request would be the better option to request Hello Games to offline players to participate.

Does that mean we get them offline if they are completed in the future?

Doubtful.

In the interim, just use the save editor to give yourself the quicksilver.

The answer to your last sentence is: NO, I've never seen CR missions turning into normal missions.
While you can do every Nexus mission alone, all missions that reward Quicksilver will require internet connection - which is not the same as playing Multiplayer which you still can turn off.

There is only 1 method to earn Quicksilver offline legitly, go to Nada and Polo and depending on your game state they may give you up to 500 QS, which really is not much.

If you have a smartphone and your PC or laptop has a network card, then you can have online connectivity on your PC via the smartphone. The mehtods are: USB tethering (connect your smartphone to PC via USB cable), Bluetooth Tethering, Personal WLAN Hotspot on your smartphone, or WLAN via a portable router with your SIM card installed (in which case you cannot receive phone calls on your smartphone).

i found that you can also play Quicksilver (daily) missions if you use the SaveGame Editor to enable them on an offline-only savegame.

Really? You make me curious now :)
Can you tell me the JSON category (left side folder), or entry? Thanks!

In another thread you also mentioned using an editor to activate items in the quicksilver store. I would also love to hear an explanation of how to enable quicksilver missions and items in the quicksilver store using an editor, since I only ever play offline. Also, are you referring to NMSSaveEditor or another editor?

https://youtu.be/JhbQaX9f8AM https://youtu.be/JhbQaX9f8AM

That's awesome good information, thank you very much! I hope this works also because a lot of people playing in Windows 7 have problems to get these Quicksilver missions.

I use Windows 7, too :)
So it definetly works with win7 :-)

This is one of the main issues with NMS, some items require you to go online, which is impossible on my offline gaming pc.

you can follow these instruction to get online-only items on an offline pc:
https://www.youtube.com/watch?v=JhbQaX9f8AM&feature=youtu.be

Those items were included in 1.58 which was released on 30 August 2018.