toNested.js

const data = {'a.b': 1, 'c.d': 7,  'c.d': 9}

const assocPathDataLast = curry(
  (mergeAble, pathList, val) => assocPath(pathList, val, mergeAble)
);

const processKeys = compose(
  split('.'),
  head
);

const expandKeys = compose(
  apply(assocPathDataLast({})),
  juxt([processKeys, last])
);

const toNested = compose(
  mergeAll,
  map(expandKeys),
  toPairs
);

toNested(data)