typedef struct { int x, y; float hello; } Foo ;

Foo*  asFoo(ENTITY* e) {
  Foo* r = (Foo*)(&e->skill[70]);
  return r;
}

typedef struct {...} Bar;
const int BAR_SLOT 70;
LIST barList; // any dynamic list implementation will do here, i've once contributed one, just adapt the functions' names below

Bar* AsBar(ENTITY* e) {
    if (!in(barList, e)) {
        e->skill[BAR_SLOT] = (int)malloc(sizeof(Bar));
        push(barList, e);
    }
    return (Bar*)(e->skill[BAR_SLOT]);
}

...

void cleanup(void) {
    for (i = 0; i < size(barList); i ++) {
        free((void*)pop(barList));
    }
}