Putting gas extractors on your exploration ships doesn't allow them to use the gas (in fuel cells) it just has them store it (in cargo bays). To use it, they would need to dock with another ship of the same design. So you would basically need two ships to do the job of one.

For long distance (i.e. endurance) scout ships, think seriously about putting a gas extractor (or mining engine) aboard the ship (design). That way, if they run low on fuel, they can visit a nearby gas cloud or gas giant planet that has the fuel type that the ship is using.

The problem with using constructors as the base design for this refueling role is that they are built by Worlds and NOT by the Starports above those Worlds, so this ties up a chance to build a real constructor or colony ship at that World.

I have experimented with trying to use warships (explorers should work equally well) as substitutes for private mining ships. I know that it is possible to give the Mine at this location order to these ships.

I also tried adding a gas extractor to the design of exploration ships as well as a docking bay, although you may have to dock woth another of your own explorers in order to transfer fuel from the cargo bays of the first ship to the fuel tanks of the second ship. If it works properly, it adds more effective range to your exploration ships (this is a variant of the ideas above).

I did this with constructors, when I had a big fuel shortage, they usually have a ton of cargo bays already so I refitted them with 1 or 2 gas miners each and had some of them mine fuel for a while.

Oh and I read there's a cap on gas mining rate - just put 2 gas mining components max (3 ore/luxury, a 4th will only add 10%,).

Yes, I believe you could, but I haven't tested it.

Couldn't you use a constructor ship as design, so you could have more room (builder ship can be the size of station), and have it do both refuel and repair at the same time?