you cant really, even some of your Under 18's will play for the reserves, the only way you can do this is if you:
1) Take control of your Reserve team (you will have to use your reserve players as you wont be using your 1st team players to play the reserve set and mandatory Reserve games)
2) You dont have to accept the Reserve team invitation at the beginning of the season
3) not too sure what the question is asking exactly to be honest, but I hope I have answered what you were asking....