apt-get command not working

[pranita-mindstix]

I have followed the steps for compilation and successfully booted for the raspberry pi, but now want to install some packages through apt-get so the command is not working, so my question is will it be possible for me to modify the config file, buildroot-/configs/raspberrypi_defconfig, so that I can add my own dependencies which I am intrested and if it’s possible then tell me the solution for the same, through which I can add the contents for modifying this file?