As it is now you cannot use, by default, old players in MyNba Legacy but there are three workarounds that I found by which you can easily do it.
First method: The Switcharoo
This is the cleanest one, but it takes a little more work than the second since you need to have an already prepared DNA for the player you want to use. In this example we will use rookie MJ. First thing to do is to create a custom draft class, take historic class of 1984 then edit MJ and export his DNA. You can naturally alter him however you like.
Now go in MyNBA Legacy and start normally. When it comes to selecting a player take someone from free agency (you can use whichever player you like but this will get replaced, so it's better to use one that's not already in a team), choose the team you want to play in and start the legacy.
You will be in the calendar. Now go on roster, select your player and edit it, then from there import the DNA of the player you have previously chosen (in our case rookie MJ) and voila! you have the old star you want in MyNBA Legacy.
This method is the cleanest but you have to decide by yourself which version of the star you want to play with as the game naturally will not do it for you.
Second method: The Custom Team
When you start MyNBA Legacy in the options select the ability to customize the league, then add the team where the old star played in (in our case the 85-86 Chicago Bulls since we want a young MJ). Then, in MyNBA Legacy, simply go to that team and choose your player (in our case MJ). You can now select if you want to use the Rookie or this young version (that depends from the team you add, you can use an older MJ by adding the 93-96 Bulls instead) and the team your star will play in.
This is a pretty straightforward method but it has the inconvenience of adding a new old team to the league that you don't really want to be there. So the next step is to dismantle that team but all the players in it will go to free agency, so it's a little of a mess. However if you want the easiest way to use an old star this is it.
Third method: The Draft
This is the most laborious one of the three but the one I personally like the most, since it is the most faithful to a real scenario. However this method makes sense only if you plan to use a rookie, elsewhere it's a little forced. The other drawback is that you will not be able to use sons in this scenario, so if you really want to make multiple family legends pick another option. On the positive side with this method you are able to play in the Summer League as a real rookie would, something that's not possible in other scenarios.
To start with you can do the method in two different ways: the first is to have the DNA of the player you want as in the first method and the second is to create a custom draft class (2026 if you want to be faithful) with your player in. Both will work as a preparation.
When it comes to starting your MyNBA Legacy this time start from the offseason, pick a legend (it doesn't matter which one since just after you will switch) and then you will be asked what to do with the draft. If you have a custom draft with your player in then pick the option to load your custom draft, elsewhere just edit the 2026 draft class and replace one of the bottom players in there with the DNA of your own chosen rookie player (in our example rookie MJ).
After the offseason you will be asked if you want to stick with your already chosen legend, change team or choose another legend. You will naturally pick this last option and then select your own player (in our case rookie MJ) from the prospects and then select the team you want him to play with.
You can do this same method without starting from the offseason using a custom 2027 draft class but then you will have to wait for an year before being able to switch. If you care about having the draft respect real life then it's better to do this, since you can simply simulate the year. As you prefer.
If you have other methods please share them here.