well usually u don't use keil to download hex files to your chip....
But any little programmer. I don't know about atmel, but for exemple u have some 8051 by philips 87C something which let u download the .hex into the onchip flash on a eprom programmer ( hi end model , unfortunately)
or mainly if u don't have on chip memory, u gonna use external memory, and in this case u will again use a memory programmer
This not exactly correct the new ver of Ke*il (7.02) has the possibility
to add a flash tool into the project. The call to this tool is configurable
(Options->Utilities) So you can start the the programmer within the ide.
You can use ISP in circuit programming on this chip, you just need the hex file. There are several designs for simple printer port programmers, you should find plenty on the net, along with the relevent programming SW. Also the commercial serial port ones are quite cheap, especially in kit form.
I suggest using some Ph1l1ps 51 parts with serial ISP and built in boot ROM. You can d/load the programmer utility free from their website. THen it is just a matter of a serial cable from COM to target board. I have used this for 89c51rd2 and found this to be the fastest way to develop where you can keep the board connected to the serial port, develop, d/load, test adn repeat in the shortest period of time.