This feature is not readily available on the UI or the API, but apparently it can be done.
It seems to be possible to link several accounts belonging to the same advertiser, and (among other things) make remarketing lists from one account available in other accounts. Enabling this involves requesting it from an account representative.
I've seen it on the AdWords accounts of a big retailer my employer has been working with. User lists from the "main" account of this retailer where available in the shared library of the other accounts and lists imported from another account had their UserList.accessReason attribute set as "SHARED" on the API.
I do not know much more about this feature, and their is no documentation about it. And apparently even AdWords account representatives do not all know that this is possible, as demonstrated the last time we tried to request it for one of our accounts.