Topping up every campaign that is running low means refilling ones that should have been stopped. The classification is the work; the building is the easy part.
The classification is the value
A campaign running low on leads is not automatically a campaign to refill. It might be one that was killed and never archived, one whose segment is exhausted, or one that wrapped when its quarter ended. Refilling those spends money to keep something running that a previous decision already stopped.
So every campaign gets classified before anything is built, and the skip reasons are written down. Reading the list of skips is usually more informative than reading the list of top-ups.
Bench first
Most engagements hold leads that were sourced, verified and never used, because a campaign was paused or a segment was reordered. Those are the cheapest leads available and they are checked before any new sourcing runs.
Attaching is the last thing
Lists are built, then shown, then attached to the existing campaign only after a second approval. Suppression is applied throughout, so a top-up cannot reintroduce somebody who opted out of the campaign it is refilling.
Runs well with
Skills compound. These are the ones we usually install alongside it.