可以用另一种套餐来加购吗？

No — a top-up always keeps the plan's original structure. If you want a pooled structure, that requires buying that kind of plan separately.

That depends on the terms each supplier sets for that plan; there's no fixed number that applies across every plan. Check your own order page directly for that.

Generally no — the existing profile receives the added amount or validity automatically. Confirm on your own order page though, since it can vary by supplier.

You'll have two profiles on the device at once, may need to switch between them yourself, and should check the device still has a free profile slot.

Not every plan does — some don't support top-up at all under the supplier's own terms. Check the option on your own order page for a clear answer.

Check the region stated on the original plan's own detail page directly. If the new country isn't on that list, a separate plan for that country or region needs to be bought.