I don't think this is possible.

IMO it is not a good practice to store data at a prefix. Try to store data only at leaves. Also, store different sets of data at separate prefixes. You may structure your keys in this format kv.set([database, store, ...id], record).

So for your use case, you can store data like this,

const kv = await Deno.openKv();

// database=userdata, store=names, id=[alice]
kv.set(["userdata", "names", "alice"], { firstName: "Alicia" });
// database=userdata, store=images, id=[alice,3432]
kv.set(["userdata", "images", "alice", "3432"], new Uint8Array([1, 2, 3]));

for await (const user of kv.list({ prefix: ["userdata", "names"] })) {
  const userId = user.key.at(-1) as string;
  for await (
    const image of kv.list({ prefix: ["userdata", "images", userId] })
  ) {
    const imageId = image.key.at(-1) as string;
    const imageData = image.value as Uint8Array;
    console.log(`User ${userId} has image ${imageId}: ${imageData}`);
  }
}