Can parameters be defined in one YAML file and reused in another?

TL;DR

Is it possible to have a parameter and its values in a template YAML and have the main YAML read the list and present them in a drop-down when preparing to run a pipeline?

Very similar to the MS example of "Variable reuse", shown here.

# File: vars.yml
variables:
  favoriteVeggie: 'brussels sprouts'

# File: azure-pipelines.yml

variables:
- template: vars.yml  # Template reference

steps:
- script: echo My favorite vegetable is ${{ variables.favoriteVeggie }}.

Details

I have a list of some 50 values that grow from week to week. When an Ops person runs my pipeline, I would like them to select a value from a drop-down.

This can be achieved simply by specifying a parameter block in the pipeline YAML file(azure-pipeline.yml):

parameters:
- name: subscription
  displayName: Subscription Name
  type: string
  default: sub A
  values:
  - sub A
  - sub B
  - sub C
  - sub D

This gives me the drop-down in the UI:

In my example above, there are only four values, however, we have over 50 values (subscriptions) and growing. I am trying to devise a way where 1) Ops won't have to edit the pipeline 2) keeping the main pipeline file concise.

I know there is currently no way to dynamically populate the list of values, but what I am looking at doing is to use a template YAML file that will have just the values and it will read the template, populating the drop-down. It is then a fairly straight forward task to have a script/pipeline dynamically update the values in the template file.

Reading the docs and some blogs, I believe this may be possible, but I haven't managed to crack it.

It appears that the parameter block can take "any YAML structure", although I haven't been able to find a suitable example of this. Amongst other things, this is what I have tried:

templates/subscriptions.yml

- name: subscription
  displayName: Subscription Name
  type: string
  default: sub A
  values:
  - sub A
  - sub B
  - sub C
  - sub D``

azure-pipelines.yml

...
template: templates/subscriptions.yml
  parameters:
    subscription: []

stages:
- stage: Example
  displayName: 1.0 Demo Stage
  jobs:
...

I wonder if my issue is that I need the parameters to be read right at the start and this isn't supported. It only works in a stage/job etc.

If anyone has any ideas, I'd be grateful if you could point me in the right direction.

T.I.A.