Well done 
The board is not suitable to add a custom I2C sensor. There should be a header with a I2C bus and power. The best power is the gated 3.3V called "+3V3_GT" for a 3.3V sensor. But that signal is probably not available on a header.
If you have a module with a voltage regulator, then you can indeed use the 5V VIN signal meant to power the MKR board 
I don't even know an other solution.
The other I2C devices that are found are 0x60 for a crypto-chip, 0x6B for a battery chip, 0x22 for a I2C-expander. Everything you see with the I2C Scanner is correct 