Abstraction for included codesystem/valueset?

I have now been reading a lot about codesytems and valuesets in fhir, but still don’t know how to use them in one specific context of my application. Let me explain:

In my application I want to make use of a fhir based terminology services for managing valuesets and codesystems. Many of theses valuesets or codesystems are either defined by my organization or official sources (LOINC etc.) and imported to this terminology service by my application during installation/update procedure.
However, there are also codesystems where the concepts are defined by the customers of my application! E.g. think of the Practitioner Role code, where my customers do not accept a standard codesystem, but want to manage the codes on their own.
In my application there is a user interface for managing practitioner roles. Now my question: How should I define the reference to the corresponding valueset? Or is there a way to define a valueset that includes a relative URL to a codesystem?
In the specifications I only see absolute and globally uniques URLs, where I do not see a way to use them.

I hope my issue became clear and some of you have an idea how to manage it.

Best regards,

If an application instance is allowed to make up its own codes, then you’ll need to have a code system URL that is specific to that particular instance. E.g. If there are 30 labs using a piece of software and they each have the ability to make up their own supplemental lab observation codes, then you’ll need to have a separate code system URL for each lab for their supplemental codes. If you need a value set that draws from those, the value set will have to include the URLs for each. There’s no way to define a value set that draws from some arbitrary unknown set of code systems - though I suppose you could do that with just a hand-wavy text description rather than a formal definition.