This sounds like it would have to be a separate mode, since otherwise it would be very easily confused with scrolling which the slide gesture is already used for - and this could make selections more difficult if they're not contiguous and located far apart; compare
1. Scroll to the first item; tap to select it; scroll to the second item; tap to select it; perform the desired action with the selection.
2. Scroll to the first item; enter selection mode; tap to select it; exit selection mode (and hopefully it still keeps the selection!); scroll to the second item; enter selection mode; tap to select it; perform the desired action with the selection.
1. Scroll to the first item; tap to select it; scroll to the second item; tap to select it; perform the desired action with the selection.
2. Scroll to the first item; enter selection mode; tap to select it; exit selection mode (and hopefully it still keeps the selection!); scroll to the second item; enter selection mode; tap to select it; perform the desired action with the selection.