You can set Acquisition program to None during List Import.
The question is, do you know the Sources? Do you want to track each import this way? The reason I ask is if you already have these leads from SFDC and load them in via Marketo (for deduping?), then maybe knowing the acquisition program isn't a good idea.
Acquisition Program means that this lead is associated as a net new lead in that Program - this Program is fully responsible for the First Touch. If that's true, great. If not, then you can just make the lead a Member of the Program.