Feasibility of using with another DB backend

So, the necessary backstory is I have a big internal application I happen to support which is built on MSSQL, and it's at best infeasible to migrate it to PgSQL. I really would love to move away from the hodgepodge it is today, but rewriting 15+ years of spaghetti code (most of which I didn't even read, let alone write) in Haskell+IHP overnight is at best improbable. So the idea is to try to do this incrementally, adding new features with IHP and slowly but steadily migrating parts of existing functionality over as well. However, doing this requires accessing the existing MSSQL database at least in some capacity, if not straight up using it as a "true" backend. So the question is, how feasible is it? Do I have a chance or do I need to look elsewhere?