You cannot add "Traded Picks" until you have finalized your draft order.
If you see the red "Finalize Draft Order" button, that means you have not completed that task, which is first required before entering in keepers.

Once you have Finalized Your Draft Order, then you will be provided a new tab called
"Pick Trades":

You can always go back and UNLOCK your draft order, and then re-sort your league draft order if needed.

 However, you will not be able to enter keepers until you first finalize the order.