Google Sheets: Is it possible to integrate with an existing spreadsheet?

Can I have data sent to a sheet that is already created, or does it have to go to a brand new sheet for each form.

Our integration will create a new sheet on your Google account, but if you want to send the data to an existing sheet you could use the Zapier integration, they allow you to select an existing file on your account and pre-fill the cells with your form submissions. 

https://zapier.com/zapbook/google-sheets/jotform/ 

I hope this helps.