適当に
やり方はいろいろだと思うけれど・・・・とりあえずで

public class c_ccc
public sub xxx
end sub

public sub yyy
end sub

end class

public class d_ddd

public sub zzz
end sub

end class

public class aaa

public ccc as new c_ccc
public ddd as new d_ddd

public sub bbb
end sub

end class