"I assume you are talking about using a serial EEPROM or serial flash device, and that the serial protocol is something like I2C or SPI. If that is the case, C51 does not have support for accessing these types of devices as a native memory space like it does for data, idata, xdata, etc."

That's what I used to think, til Jon Ward put me right:
http://www.8052.com/forum/read.phtml?id=27259

As this is very specific to Keil, it'd probably be best to ask any furhter questions on the Keil forum.
(if you do, post a link here so that others can still follow the thread).