You can download a number of Yamaha Expansion Packs from the Yamaha website for your SX700. The expansion packs are free.
https://uk.yamaha.com/en/products/contents/keyboards/vse/index.htmlHowever... Firstly, you need to download the programme Yamaha Expansion Manager to your pc. Then check out your Manual for instructions on how to do it. Make sure that you download the latest version of YEM. I haven't put a link as it depends if you're using Windows or Mac, but it's available for both platforms.
You need to familiarise yourself with the program and you'll have no problem
The packs are in compressed form, so you need to download them to your pc first, and then unzip them to (I would suggest) a new folder where you can easily access them when you install them using YEM. No conversion is required. If you already have some pre installed expansion packs on your SX700 you will need to add these to your new packs, because those packs will be deleted during the installation process, and will need to be reinstalled. That's just how it works.
If you have any problems, then there's plenty of help here, but my advice would be to check out a couple of YouTube videos which cover this subject very well.
Do it methodically, and you'll have no problem.
Allow time, it does take a little time to complete the process.
Hope that helps
Keith