existing list

[Anonymous]

How to I append an existing list?

[Josh_Hill13]

Do you mean add to that list?

Use the Add to List flow action

If you are importing, you must import to a new list, then move List 2 Leads > List 1 List.

Then you could remove Leads from List 2 and delete the list.

[Edward_Masson]

Do you mean append a static list?
Have a smart campaign running with requirements that once met, will be added to your static list.