Just need to add it here: https://chrome-internal.googlesource.com/infradata/config/+/master/configs/chromium-swarm/pools.cfg#28

I misread that, you want findit task account in luci.chromium.ci bucket. that's a chrome-infra-auth change.

Added, try again.

So it turns out a task account can't belong to more than one pool, since it defeats all sort of assumptions.
Explanation here:
https://chrome-internal.googlesource.com/infradata/config/+/master/configs/luci-token-server/service_accounts.cfg#6

Can you move linux_chromium_bot_db_exporter to one of the findit buckets?

An account can be shared by multiple pools, but it is undesirable (if avoidable). Let me know if its absolutely impossible to move linux_chromium_bot_db_exporter to findit bucket, I'll help with setting up account sharing.

Another option it just to use chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com account already present on chromium.ci pool.