To piggyback off sangamc, if you want to import a pool of players and have it be so that that pool of players are the only ones included. Import them first with a projection of something absurd (like 100), then in the table, select none to exclude everyone, slide the projection filter to 99, then select all. Now all the players that you imported are the only ones selected. Go back and import their actual projections.

Side note: We're currently working very hard on prepping NBA. Once we finish, one of the things we want to do is to add an option in the player manager to zero out non-imported players.