It is possible, you need to place a file called config.json in the C:\Users\<YOUR-USER>\AppData\Roaming\Shapespark directory. With the following content:
{
"scenesDir": "c:\\Users\\<YOUR-USER>\\Desktop\\Shapespark"
}
Please modify the path in this file to point to your desired scenes directory. Use double back-slashes \\ between directory names.
After adding this file, restart Shapespark. If you don’t see any errors, you can move your existing scenes from Documents\Shapespark to the new location.