I figured out a way how u could get team stance socks for you're ballplayer in 21 RTTS.
At some point you're roster from RTTS with all player upgrades and downgrades saves to Home Menu and that's the thing how it works.
You have to equip the "The Show 21" Stance socks in you're RTTS Ballplayer Loadout menu. Than you Exit and go to Home Menu - Settings - Roster Control - Edit Player. Now you search for you're RTTS team and if it's the same as in my case you're ballplayer should be on the Team now you Click on him and chose Edit Player so you should see the "The Show 21" Stance socks are equiped. Go back and! SAVE THE ROSTER! if you saved the roster go to you're Ballplayer Loadout and unequip the stance socks. now you go back to settings - file management - load roster and go to you're ballplayers team. Edit him in there you see the team stance socks are equiped. Save the roster again and go to you're RTTS and play next apieriance and you will see the socks works