Check out the OPKG Package Manager documentation section on the `install` argument (under the heading "Package manipulation")
`opkg` can install package files from both remote and local locations, as in:
opkg install
opkg install /tmp/hiawatha_7.7-2_ar71xx.ipk
The latter is (I believe) what you want. This enables you to download the package file for `bash` on another host, put it on the local filesystem of the OpenWRT device (perhaps using `ssh`/`scp` over a direct ethernet link), and then `opkg install /path/to/package.ipk`, without having it need to connect to the Internet.