The only fittings that are approved for PEX are PEX fittings.

Even if you were to install a tee on the PEX line you are still required to have a fixture stop valve unless you have a PEX manifold and if all the valves on the manifold are clearly and indellibly labels as to what each line serves, in which case the PEX manifold valve may serve as the fixture stop, althought many local codes still require the individual fixture stops.

Your municipal supplier generally locks the box at the street that contains the curb stop valve, but code also requires a full bore stop valve on the water service line within 3' of where that line enters the structure, and that valve code requires that you must have free access to that valve.

If you are tapping into the hot water line, code also requires a full bore stop valve on the cold water line supplying your water heater, If you turn that valve off the hot water is shut off throughout the structure.

So you can use either your main water shutoff valve or the water heater shutoff valve to turn the water off, then just remove the angle stop valve and install a side port angle stop to connect your dishwasher.

"Your municipal supplier generally locks the box at the street that contains the curb stop valve..."

The only time I have seen a lock in a meter pit is when service has been turned off, the meter removed, and lock used to prevent a meter from being installed.

Did I say anything about a meter pit?

Meter pits are used in areas where the climate is not subject to deep frost, however in colder climates the meters are always installed inside the structure to protect them from freezing.

When a municipal water supplier establishes a new service the place a saddle tap on the water main, then run a short line to a "Curb Stop Valve" which is installed on the line well below the frost line. A pipe sleeve is then placed over the curb stop valve and extended up to finished grade, where a lackable cap is put on the top of the sleeve.

To access the curb stop valve you must first have a special 5 sided socket that will remove the curb stop box cover, then you insert a curb stop wrench down the sleeve uptil it contacts the curb stop valve. The curb stop valve rotates and it has a flat bar instead of a common valve handle. The curb stop wrench is a 3/8" steel shaft with a horseshoe shaped piece of flat steel on the bottom end that will straddle the bar on the curb stop valve to turn the valve.

Generally the municipal water main is in an easement on the side of the street and the curb stop valve is typically place in the easement or sometimes it may be in the sidewalk, but in all instances it will be between the street and the house side of the sidewalk.

Generally you will see a similar cap that is marked "Gas".

Contrary to popular opinion it is the curb stop and not the meter that defines the official point of demarcation between the municipal supplied line & the customer supplied line.

In the warmer regions were they can use meter vaults the curb stop valve may be placed in the meter vault.

In colder climates where the meter is placed in the house the customer is responsible for all pipe from the curb stop, and the water supplier will instruct the plumber where to locate the meter bracket in the house or they may just tell the plumber what the meter allowance is, and the plumber will leave an open gap in the line, where the water supplier will later install the meter after the plumbing has been inspected.

And the ones in every area I have lived do not have locks.

They are likely relying on the fact that a long T-wrench is needed.

Many places do not even have a stop, but place it in the meter pit.

For interior meters the stop is often at the property edge, but not locked.