PCS I believe sell any of the components separately to an existing customer. You can certainly choose upgrade from your account and buy more ram, new HDD etc. so Memory Card reader should be doable. The only thing I would say, the reader is usually pre-populated in the config so the only reason I can see it wont have been is if the case you chose does not have space for or does not support the reader.
Best thing, ring them tomorrow and ask them, if it will go in your case then you can probably just order it with them there and then on the phone.
Best thing, ring them tomorrow and ask them, if it will go in your case then you can probably just order it with them there and then on the phone.