If Tradfri bulbs use ZLL, is it theoretically possible to connect Tradfri bulbs to an existing Hue network?
However Philips appear to be modifying Hue software to prevent it. Cloud lock-in. https://developers.meethue.com/comment/2337#comment-2337
Philips Hue hubs do work with 3rd party ZLL endpoints. However, the likely reason they disabled 3rd party device support was because all Zigbee products have compatibility issues; nothing works as well as staying with one brand.
You can't always control all features and can't update firmware of all 3rd party devices from any hub. Sometimes you just need to manually configure the drivers (in code).
As a side note, IKEA's brightest bulb is 1000lm which is much brighter than the brightest Hue bulb, and it's cheaper :) I can perhaps see why Phillips would not go out their way to resolve this issue.
[1] https://developers.meethue.com/comment/2686#comment-2686
Edit: fixed typo of watts vs lumen :)
1000 Lumen. 1000 Watt bulbs exist for grow lights but IKEA doesn't make them that thirsty.
What I believe you are describing would require a ZigBee radio and support for the ZigBee Light Link protocol (or maybe ZigBee Home Automation?). Luckily, there's a PR being worked on to support that in Home Assistant: https://github.com/home-assistant/home-assistant/pull/6263