Is it sensible to add steamed vegetables to a vegetable curry to reduce cooking time?
I‘m sure you could do this.

But the gain may be less than you calculate and the results will not be exactly the same:


If you are (pre-)steaming the vegetables, your are keeping their individual flavor whereas by cooking in the curry sauce you get a more evened out flavor as the various ingredients contribute to the overall flavor and absorb the spices. This may or may not be what you prefer, but it’s how your first recipe is designed.
If you are using a pressure cooker, you might reduce the overall cooking time, but you are doubling the washing up: two pots instead of one. And most pressure cookers need hand washing.


Another aspect:


If your vegetables took significantly longer than you expected, it’s likely that you used bigger chunks than the recipe author. So if you want to save cooking time, consider cutting your vegetables smaller. Unless your stove has serious problems, the amount of veggies shouldn’t be too much of a factor.


Bottom line:
It’s up to you which approach you choose to reduce the cooking time, as both can work. I personally would just cut the vegetables a bit smaller.