All 50 free agent signings must be done in the same Career so they don’t carry over from save to save. When you are in the Career Mode menu, scroll down and click on Transactions. Now scroll down more and click on Free Agents. You can sign any player that you want. The fastest way to get a free agent to sign with you is to choose a bad player (someone with a low overall rating) and offer him a contract for $1 million. Once you offer the player a contract, go back to the Career Mode menu and click Advance to Next Day. You should get an E-Mail when the player accepts your offer and a popup should come up saying that you need to move the player from your Pending Transactions to a roster.
From the menu, scroll down and click on Pending Transactions. You should see the person you made your offer to on the list on the right and your roster on the left. Press to switch to a minor league roster. Scroll down the roster list until you see an empty slot, press over the free agent’s name and then press again over the empty slot. Do this 50 times to get the achievement.
It is best to go for this while you are going for the 30 year career achievement because you will have a lot of time to get it. You don’t have to sign bad players all the time, when the season ends, there will be a lot of good free agents available so you can try to get some of them. Try increasing your Contract Negotiation stat to make it easier to sign free agents and picking a team with a lot of money, like the Boston Red Sox, New York Yankees, or Chicago Cubs, so you have a high payroll.
When you run out of room on your MLB roster to place the players after you sign them, press to go to your minor league teams. Once you fill up your next roster, move on to the next one.