If you pass the VISA resource name to your DLL as in string format, your DLL must separately open an alternative VISA session to acquire the new ViSession value. In the DLL, you can then access viWrite/viRead with the acquired ViSession, then close it when you have finished to use.
Another way is directly pass the ViSession that is already acquired in your LabVIEW app to your DLL. You can use the same ViSession without having to open/close alternative VISA session.