the only way i can think of doing it is when you're picking the leagues at the start,make sure you pick all the B leagues, then on manager options (where you can let your ***.man take charge of friendlies etc) click take control of B team if its possible
I dont think you can do it as its not like a reserve team, its actually another 'club' so to speak, playing in an active competetive league. It would be like Alex Ferguson managing Oldham as well as Man Utd. Or something like that.....