Our developers add features to our managed packages using SFDX and scratch orgs, and our Continuous Integration uses those too via https://claimvantage.github.io/sfdx-jenkins-shared-library/. Scratch orgs support namespaces.
But at the moment, the package version upload is done back in the old developer edition org by doing a:
sfdx force:source:convert
followed by a:
sfdx force:mdapi:deploy
then followed by a manual version upload. Not too big a burden, as for us this is typically the final step after several weeks of feature development.
Eventually, packaging 2 should simplify this and make it more automatable.
PS
This single command:
sfdx:source:deploy
can be used in place of the two convert/deploy commands.