Linked repositories can be created out of Java Specs as reported here:
But we can't export a Linked Repository configuration to be reused or customised later.
Any release of Bamboo
Add an option to export a Linked Repository to either Java or YAML Specs format. The exported code should be able to recreate the repository, including its permissions/ACLs, Specs Repository access, Bamboo Specs status (enabled/disabled), and Repository access settings