It can't, and I wouldn't hold my breath for such a small company being able to navigate compliance for contactless payments. The Pebble does use standard watch straps though, so you could get one of the ones with a programmable payment chip embedded inside.

If that's all you use your smartwatch for, you may as well skip the watch and get a payment bracelet or ring though.

e.g. https://www.curve.com/wearables/