How to create reactivity outside .svelte files with a spring store?

[205g0]

// within <script /> of a .svelte file!
...
let someSpring = spring(0)
let someStore = derived(...)
$: $someSpring = $someStore
...

Now, whenever I change $someStore, $someSpring gradually changes the value as it should and you can use it for butter smooth animations in the markup below.

How can I get those three lines in a separate .ts module which I can import in any .svelte-file but still nicely working? I tried something with derived (see below) but didn't get it working and had hard transitions, only.

PS:

• someStore is required to be a store for my use case, my example is simplified
• what I've tried:

// in a .ts file
export const extSpring = () => {
  let someSpring = spring(0)
  let someStore = derived(...)
  return derived([someStore], ([$someStore]) => {
    someSpring.set($someStore)
    return someSpring
  })
}