Edit Content of IPSW file to install older baseband firmware
Downgrading the baseband hasn't been possible to do on the iPhone 4 since iOS 4.0.x. Even if you had been preserving the baseband, the tool to do this with (sn0wbreeze) hasn't been updated since iOS 6, so you wouldn't be able to do it anyways. And to answer your question directly, no, it's not possible to downgrade it by injecting the older firmware into the IPSW. It kind-of used to work that way, but things have changed drastically since iOS 4 (and you needed a bootrom exploit to do it anyways on iOS 4).