Lets assume you have zip files and you have uncompressed txt files in your folder.
First step is to create a archive compressing txt files using following command.
you can use -m for setting levels of compression , this also depends on compression method you are using
Lets assume here to be default compression only
7z a compressed.7z *.txt -r
You will get compressed.7z now.
The second step will be to copy the already compressed file to this archive or create a new archive
7z u compressed.7z -u!update.7z -mx0 *.zip
There is further elaboration of update command here
And in case you have liberty to use graphical interface, the update can be done in a following way