Stellaris – How to move the civilian (scientist) ships to other empires territory

There is little you can do at the moment, it may be added in a DLC.

If they are rivals, they hate each other. Influencing how somebody feels about a third person was never something you could do a lot in Paradox games, but a feature like this may be added, when diplomacy gets expanded in a later dlc.

The best shot you'd have is to declare war on a rival of your potential ally. This may cause your ally to declare this empire its rival, with some luck your ally removes your potential ally from his rival list to do that and combined with the mutal rivals opinion bonus, that opens up the remote possibility of your ally and potential ally reconciling their differences. But it's a very long shot.

There are a number of technologies you can research that increase the size of the borders around your outposts and colony ships, including

• Galactic Ambitions

• Manifest Destiny

• Propaganda Transmissions (Repeatable)

The Stellaris Wiki entry on borders also says that:

Border spread appears to work volumetrically. That is, as the empire becomes more dense and grows, its borders also appear to spill outward. Removing colonies or frontier outposts from within the empire's well-established borders may affect the outer border unpredictably.

So building additional colonies and outposts in your empire's interior can increase your borders at the fringes.

If you're expanding your empire via frontier outposts, it's worth noting that

Outposts controlled by sectors do not cost Influence to maintain.