How to extend a live cd/usb?
 

My case is I love Grafpup. but I miss emacs.
I guess similar cases exist where users want a specific program
which is missing in their favorite live distro.

I have a debian install on my hard disk but need a live distro on my usb stick when I am mobile. Currently I use Grafpup.

I would like to add emacs and texlive to any small live distro.
I could just download and use the 700Mb live usb images, but then, I need space for my data too on my usb stick.

I can spare a about 400 Mb partition on my stick for the Os.

I think you want to look into "remastering" on google. I've not ever done this before other than opening the ISO image before I burn it and adding a few standalone applications. It may be more difficult if your app requires certain dependencies.