New Wall Sconces
you would need to interupt the power going to your 2 sconces via a single pole switch.
(assuming one switch for two lights)
then you'd possibly need to 'fish' a wire to the new switch location, assuming it is not proximal to said sconces , this may be a job in itself.
if there is a possibility that the sconce wiring is viewable from your basement, and verified carefully as being just that, gettting from point A to B becomes a little easier.
much depends on how tight your home is...
another option, although somewhat technical , is X-10 switching, either way i suggest that it may well be worth your time to have someone knowledgeable in such matters to take a look and help you decide.