It's certainly possible to start with lap belts and add shoulder belts later, although it means you'd end up having bought two sets of belts. I disagree that it's "not a lot of extra effort" to add shoulder belts – in my experience, the shoulder portion takes at least 50% of the effort if not more. Care must be taken to identify or construct a suitable attachment point. It will be near the visible parts of the car, so there may be aesthetic concerns.
But I also agree that it's worth the trouble if your intent is to protect the human body in an accident. Many folks argue against shoulder belts on the grounds that the attachment points will always be weak. This may be true, but there is a wide range of accident scenarios forceful enough to injure or maim, but not forceful enough to rip the seat belt from its mounting point. Adding a shoulder belt adds protection in this zone of accidents.
However, if an accident is severe enough to cause the shoulder attachment point to pull out, this would render most shoulder belts worse than lap belts, since the detached top belt would just run through the buckle unrestrained. To correct this problem,
when I put 3-point belts in my coupe, I added a metal clip, through which I routed the webbing above and below the buckle. This locks the buckle in position, so that if the shoulder portion were to fail completely, the lap portion would still hold. It's a bit annoying to reposition when we change drivers, but it was the only solution I found that eliminated this risk.