Forwards And Sideways In Time

The Technic History and the Time Patrol are not connected but the Maurai History and There Will Be Time are, so Poul Anderson did interconnect a future history series with a time travel novel. Notionally, time travel provides a route between alternative future histories, provided that the past is mutable which, in There Will Be Time, it is not. However, I am now inclined to think that, by travel through time, we mean travel from our present to our past, thus that anyone who enters an altered timeline is in this sense no longer a time traveler.

Time travel is a possible if infrequent occurrence in some future histories:

Anderson's Maurai History;
Asimov's Robots and Empire History;
Star Trek;
Heinlein's Future History, if we include Time Enough For Love (I don't);
Aldiss' Galaxies Like Grains Of Sand;
Niven's Known Space History.

Kzinti discuss other species that they have attacked:

"'And that funny religion on Altair One. They thought they could travel in time.'
"'Yes, sir. When we landed the infantry they were all gone.'
"'They must have all committed suicide with disintegrators. But why? They knew we only wanted slaves. And I'm still trying to figure out how they got rid of the disintegrators afterward.'
"'Some beings,' said the A-T Officer, 'will do anything to keep their beliefs.'"
-Larry Niven, "The Warriors" IN Niven (Ed.), The Man-Kzin Wars (London, 1989), pp. 5-26 AT pp. 18-19.

All that the Altairians needed to do was to travel forward in time until after the invaders had gone. This means that time travel is possible in the Known Space universe and that Poul Anderson or others could have included it in their Man-Kzin Wars contributions, provided that it made a good story, of course.