It would not be allowed by most plumbing codes but you could put a one way inlet vent valve on top of the existing vent pipe. That would allow air into the vent system to prevent traps from being sucked dry or gurgling but would not emit waste gas (or allow pressure to escape).
These one way valves are readily available to deal with situations where existing plumbing has no vent and adding one would be prohibitively difficult.
As I mentioned, though, many local codes do not allow their use in lieu of an open outside vent stack.
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