@Jellow Collect supports capture of external sensor data through the external app widget. That's how we support use-cases like fingerprint scanning and reading rapid diagnostic tests. You will have to build that external app though.
Some Bluetooth enabled devices (e.g., digital calipers, custom barcode scanners) can pretend to be Bluetooth keyboards and "type" the value that they capture. In that case a simple number widget will work just great. No additional software development needed.
If the above ideas won't work, you could also take a picture of the scale and use on-device on on-server vision recognition to read the value and add it, along with the picture, to the submission. We've done something similar with audio transcriptions.
Tell us more about the end-to-end workflow you want to support and why capturing weight is important. What problem would having this capability solve?