how to replace a .dat file that is inside a .dat file?

[sanboo11]

Recently, I was trying to modify the th6 dialogs, for that I extracted the files from "th06e_ST.dat" and modified "msg1.dat"

So far everything was going well, but when I want to put the modified msg1 in a th06e_ST.dat, all the files inside are simply deleted and only the modified msg1.dat remains.

I want to replace the msg1.dat of the th06e_ST.dat with my modified msg1.dat, how can I do it?

I'm sorry if my question is stupid, I recently started with this