My understanding is that in Sandbox mode,
you will need to import the players each year. The program will import many fictional players for you. The number of imported fictional players will depend on your chosen roster sizes. If you choose to go with 50-player rosters, PS will import many fictional players to make up for retirements, career-ending injuries, etc.
You should have no limitation on the number of seasons. Only in the Season Replay mode are you limited to just that one season.
The first time I used such a setup, I discovered a site called
www.randomizer.org. I used this site, coupled with
www.baseball-reference.com to decide who I brought into my association each year.
I'll give you the short version of what I would do in a setup similar to what you're going to do:
(1) Make a list of all the years you want to draw from. If it's 1950-1969, you will have 20 numbers. Write this list down: 1 = 1950, 2 = 1951, etc.
(2) List every actual MLB team that you want to draw players from (example: 1969 would have 24 teams). Number them from 1-24 (or whatever your total) and write the list down.
(3) Decide how many players from each roster you want to make "eligible". 30 from each team sounds about right. This would give you all the guys on the 25-man rosters, plus a few rookies sprinkled in. I suggest 18 position players and 12 pitchers. 19 and 11 would work well also.
(4) Use the randomizer site (or any other method you know) to select sets of 3 random numbers in a sequence. You will get strings such as this one: 4, 11, 23. You would then import a player from 1953, from whichever team you've numbered as Team #11. You would look at the baseball-reference roster for that team and import whoever is the 23rd player down the list. In this case, you would skip the 18 batters and select the 5th pitcher on the list. Import that player into your assn.
What I did was to make dozens of strings in advance, then go to baseball-reference and write the player names down next to the string. Once I had all the names, I would go back into the PS program and import them. On the Import Players screen, make sure you have chosen your correct preference for "Allow duplicate players". Back when I did mine, this wasn't even an option, so I never had to worry about it. Whenever I would try to import a players who had already been in the assn, the program would not let me. This was very handy after about 15-20 seasons, when I could no longer rely on my memory.
Some of your random strings may not be valid. For example, I was importing players from 1969-2004. Occasionally, I would get something like: 1973, Tampa Bay, player #17. Of course there was no 1973 Tampa Bay. I just scratched that string and moved to the next one. You'll encounter this issue with a few of your teams, if you're using players from into the 60's. You have the Colt 45's, Twins, Angels, and Mets coming in as expansion teams.
One thing you have to take into account is the MLB year of the player you're importing. If your string gives you, say, Ted Williams from 1960, you'll be importing an old player about ready to retire. After some early trial and error, I ended up using the string to point me to the player, then used the Lahman dB to import whatever year would reflect that player's first legitimate number of AB or IP. You could set your own parameters, of course, but a minimum of 100 AB or 40 IP worked well for me. If you simply imported the player in the year of their first appearance, you would see many of them import as scrubs and never amount to much.
On this board, I have suggested adding an "Import Random Player" function. This feature would employ filters for position, LHB, RHB, LHP, RHP, and teams/years. I've never read any feedback from anyone else on this, so I don't know the level of interest in such a thing. It would most certainly benefit you in this situation. I'm certainly no programmer, but something like this seems "do-able". Any thoughts?
Have fun and let me know how this works for you.