The Wilkins water pressure relief valve at the inlet to the house supply is dripping. Can I safely plug the outlet where the drips are flowing, until I replace the entire valve? Is the valve strong enough to hold pressure once it is plugged?

No, You cannot. Dripping Pressure Relieve valve usually indicates that your pressure regulator is not functioning properly releasing more pressure to your house plumbing than it should. It is also possible that somebody opened it and it never closed back all the way. In either case, investigate what's the cause, deal with the problem - and install new pressure relieve valve. In most cases, you cannot reuse the old one after it started to leak.