Modifying smart watch or activity armband with further sensors
there are a few DIY, open source armband or watch models like :

https://www.tindie.com/products/ttgo/t-wristband-diy-programmable-smart-bracelet/

which has the sources and tutorials on :

https://github.com/Xinyuan-LilyGO/LilyGO-T-Wristband

it's based on ESP32 and is Arduino ready.the space constraint may not allow for adding a new sensor, but it already comes with a few onboard sensors.
after getting along with ESP32 and the TTGO libraries (which are all provided by the manufacturer LilyGo) that wouldn't be too hard to design a custom band.