I don't know that you can force SW to use more ram. I have seen my ram usage go to 14.2 Gb total system when I loaded large models.
SolidWorks is using as much as it needs. If it needs more it will use it. I have opened up models that used 20 gigs of RAM.
You might want to ask this in the Simulation forum. It may be that Simulation has some options that control memory usage, since memory was a big problem for simulation programs in the old days. I know that ANSYS sometimes seems to start using swap space even though I still seem to have lots of memory available. I haven't followed my own advice and tried to find settings that I can change to improve the performance.