I believe you can get into an engineers menu and manually tune in a channel as long as you know the PID/FREQ/POL/..etc... (see lyngsat.com). Just don;t ask me how to get into the engineers menu though as it changes from box to box...
You're better off with a FTA box as that'll pick up all of the sky channels and all of the freesat channels (though it will not decode sky channels unless you have an already activated card and then only what the card is registered for) but there's very little left that the sky FTV card decodes that isn't denoted as a freesat channel..
One note if you get a FTA box that has a card slot then you can NOT activate a sky FTV card in the slot but instead you need an official sky box as the firmware does the activation
After activation you can then use it for the same channels in the FTA box slot