Is integer factorization reducible to subset sum?
Yes, such a reduction exists.  Subset Sum is NP-complete.  FACT is in NP.  Therefore, by the definition of NP-complete, there exists a reduction from FACT to Subset Sum.

To find such a reduction explicitly, work through the proof of NP-completeness for Subset Sum; it will describe such a reduction.  (Implicitly, we get a reduction from FACT to SAT by Cook's theorem, and from the proof of NP-completeness for Subset Sum you will be able to extract a reduction from SAT to Subset Sum.  Now compose those two reductions.)