Reason 5 can Record samples so yes you could do it but it would be a pain in the ass to do so vs just paying the extra for Record or rewiring to a DAW and working that way. I would advise not even buying the hardware unless you have it for a specific purpose.
What he means is you cannot sequence the notes from your keyboard into Reason. You can only Record audio into Reason and Record. Being that you are misunderstanding basic concepts of production it may be better for you not to buy a Fantom. Just work with Reason for a while