I generally recommend not to import any lists directly into the database, but rather use Programs and import under the Membership tab.
This will prompt you to specify a status for the imported leads in the program, and they would all become members. Newly generated leads would automatically be flagged with the program name as the "Acquisition Program". This allows you to include the List Program into your First-Touch (FT) Analysis reports.
As to the channels - you can create yourself a generic "List Import" channel, or a special "Sales Generated" channel - it depends on how you want to filter your Analytics reports.
These channels would have a basc "Member", or "On List" status for all the newly imported leads.
Some clients insist on tracking when these leads engage with any marketing activity, and set a success status when that happens. The idea is to gauge the quality of the list.
However, I caution you that this skews your Multi-Touch (MT) Analysis, as the leads would reach succes in two programs at the same time: The Marketing Activity Program that they engaged with, and the LIst Import Program, which has acquired the lead but not contributed anything to the influence.
Hope this helps.