Set Up Secure NAS With Two Networks And One Of Them Open?

One project I want to do is get them a network storage drive (Except, due to budget reasons, all I get is the 1Tb external they've had for a year now, and a NAS adapter). I can set up the NAS no problem, but here is the catch. From the ISP we have a B/G Router/Modem combo, and for range, we have a d-link B/G/N router.

I would like to have the ISP's G network open to cover our lobby for free access, and have the D-Link's N to cover the far offices, WITH access to the files. Now the issue, how do I set up the D-Link router, to only share the NAS files with devices conected directly to it.