There may be a way... but even if it would work it would still involve a lot of time and effort as well as storage space. I play fictional more than anything else and I am dying for the day in which OOTP allows teams to compete in multiple competitions in one calendar year, so I've thought of many different possible ways to
try to make it happen in game
For your idea what you could do is save each season of your fictional league as its own file. So 2016 season ends, make a copy of the league file and name it, say, fictional league 2016. You would need to make a template, saved
in game, with say 2 blank teams. I am pretty confident the game requires a team to exist for the template to load properly. Make sure to adjust the settings to meet your preferences. You can release the players and delete the free agents and remove the team names before saving the template
So lets say you have ten seasons saved up. Open fictional league 2016 and add your template. Once loaded delete the template's teams and all league history and move the teams(s) you want to compete in your all-time league. Save the template. I think you will have to create a new name for the template. So start with all-time league 1, then all-time league 2 ... all-time league 10 etc. or whatever naming convention you prefer, of course. And you might want to delete the template from your league file after it is saved, or close the game without saving the file
The newly saved template will now have your all-time teams added from fictional league 2016 and you can repeat as needed with the remaining saved league files until, after several versions, you should finally have all the teams you want added
As NoOne stated previously there would be an age issue. Ten years wouldn't be so bad but 25 or 50 would clearly create a problem. So you would likely need to edit player ages as well. I'm not sure what would happen with player or team ID's, I would hope that if you're pulling teams and players from one league that the ID's would remain consistent and not present a problem
I have never tried this, so maybe it would collapse and never work. Maybe the template file wouldn't work. But I think the process of adding teams through the league file/template process ought to work and best of luck with whatever path you try