Actually, the solution wasn't even that hard afterwards! It is right (
@Alexandre Collin) to note that the MultiConverter Tool is doing its job. Only when there is no cooperation from the supplier do we have to get creative. Looking at the Solid Works import options, IFC also just appears to be listed. Only, an IFCzip is not. Requesting an IFC from the supplier then seems like the simplest solution, especially if files need to be exchanged more often. In this situation, the expectation is that it will only be needed once. The IFCzip file is extracted using the standard windowns UNzip functionality. The IFC file then becomes available and the processes run again.