Can I charge an iPhone 13 pro with an XR block?

There's no reason why any iPhone can't be charged with any compatible power supply, although many would recommend using one that's Apple (Made for iPhone/iPad - often called "MFi") certified. I believe the XR was packaged with variations on the 5W power adapter worldwide, which many in the US (and other countries with two flat blades) call the "Cube".

It won't be ideal since it should be capable of charging at a much higher rate than 5W, but it should work without damaging the device.

You can charge any iphone with any USB block. You may have to use the XR USB-A lightning cable.

However you will only get fast charging with a USB-C block and the cable that came with the iphone 13