In theory, it is possible to assemble a working FSO install out of files released for free, sans certain sounds (Inferno offers a decent selection of these).
In practice, you'd have to be an experienced modder to pull this off, familiar with which mod contains what (which means you'd most likely have FSO already), there are mods which wouldn't work and SCP team wouldn't support whay you'd be doing, meaning that you'd have to be familiar with debugging. And while you'd be able to run mods, certain things just wouldn't "feel" like FS with different sounds, music and no VA.
In short, more trouble than it's worth, expecially if you can just get the game for 6$ and don't live in a country that has currency with insane conversion factor to dollars.