Me again...

Now about your problem...yes...all images take entire disc space...so even if the software or something else that you're burnign is i.e. only 10MB...the image burning process will make the rest of the disc unusable for later sessions.
So...you didnt said which software did you use when trying to add more data onto same disc (image)? I guess you cant do it in some Nero-alike burning software...sooooooo...to do that you may use (at least I use this one for such cases - although there are many other such programs that would also do the same just as good

)
MagicISO...so there...you may do almost whatever you want with images...add files, delete,etc..etc...and burn them later....

So you easily open this ISO in MagicISO....then drag&drop files you want to add from desired location to that ISO...and finally save that new iso...and its done...files are added...then just BURN...and ENJOY!
Well thats how it goes at MagicISO...but I guess other members maybe use other programs for such issues....so please drop by and help toni bui with your advices...
Have a nice day/night....
SiliconFreak.