You can definitely use your original battery as can external one to up the effective Ah. Both have the same voltage, so if you connect them in parallel, you’ll end up with 41 Ah, effectively.
You’ll have to make / buy a parallel wiring harness to get both batteries connected, but it should be fairly straightforward.
When you initially connect the batteries, make sure they are both fully charged, otherwise the more charged battery will discharge into the less charged one.
If you do end up going for this, make sure to give us an update!