Given the differences in both how you structure your data and how you build objects / expressions, I would strongly advise you build everything from scratch in Qlik Sense even if there's some sort of solution out there to convert existing reports. What can you re-use, at least in part, is the underlying queries / data source connections which should remain fairly similar - but even with that, you may need to make significant changes due to the differences in how Qlik and Cognos approach data modeling.
