Converting a Mixed SUBSET-SUM Problem To All-Positive Case
Both variants are NP-complete, so such a reduction surely exists: the definition of NP-completeness guarantees it.
If you want an explicit reduction, you can reduce one to the other (reduce to 3SAT using Cook's theorem, then reduce 3SAT to the other using its NP-completeness).  The resulting reduction will be ugly, though, so I suspect this might not be what you're looking for.
If you want a simple and natural reduction, I'm not sure how to achieve it.