何がやりたいのかよく判らんが
↓これじゃだめなの？

typedef struct
{
int n1;
int n2;
}N;

func(int n1, int n2)
{
int BB;
BB=n1+n2;
return BB;
}
int main()
{
N abc;
abc.n1=1;
abc.n2=2;
int BB;
BB=func(abc.n1,abc.n2);
}