In most cases you can. A walk in shower takes a door later if the opening gives the hardware something to hinge to, either a wall return or a fixed panel set to carry that load, and if the entry has a curb or a floor pitch that keeps water inside. Sometimes the existing panel gets replaced rather than added to.
A standalone screen is engineered for its own weight and nothing else. It may sit in a channel, on clips, or in a shoe, and its thickness was chosen for a piece of glass that stands still. Hanging a door off it with glass to glass hinges loads it in a direction it was never asked to work in, so the panel thickness, the anchorage and the blocking behind the tile all get verified before anyone talks about hardware.
Where the original screen was set light, the honest answer is a new pair of panels rather than a retrofit onto glass that will complain about it.
There are three places a door can hang. It can hinge to the wall on the open side, which is the cleanest option and depends on studs or blocking behind the tile. It can hinge glass to glass off the fixed panel, which puts no new holes in the tile but leans on that panel being up to the job. Or a short return panel gets added to create a hinge point where the layout never provided one.
Then check the arc. A shower door swinging out has to clear the vanity, the toilet and the bathroom door, and the model codes New Jersey follows want it opening outward.
It is worth asking what the door is actually for. If the complaint is splash reaching the floor, a longer panel or a small return wing usually solves it with no moving parts and nothing new to clean. If the complaint is cold air, a door alone will disappoint, since warm air rolls out over the top of any opening that stops short of the ceiling. Holding heat means running glass to the ceiling with a transom and sealing the perimeter, which is a steam style build rather than a door added to an open screen. It is also worth asking who uses the shower, because a door that has to be pulled open across a wet floor is not automatically an improvement for an older parent or a young child.
Yes, with attention paid to the floor. A curbless entry relies on pitch and a linear drain to keep water where it belongs, so the door carries a sweep at the bottom and the threshold line is checked for level before anything is ordered. Where the floor drains toward the room, a door hides that problem instead of solving it.
Somewhat, and less than people hope. A hinged door with an open top still lets heat leave along the ceiling, which is where it goes. Enclosures that genuinely hold warmth run to the ceiling with a transom panel and a sealed perimeter, and they are ordered that way from the start rather than assembled in stages.
Adding a door to a shower built without one is a measuring problem before it is a glass problem. Accurate is family owned and has been working in Bergen County bathrooms for more than forty years, and the first move is always reading how the panel you already have is anchored.
