Can I add a good quality purchased natural apple juice
to the apple juice from my own trees when making cider?
If so when should it be added, before or after fermentation?
It all about apple variety, another mental crop this year, but these types of apples ferment out so dry and have little apple profile you need bramley and bittersweets to blend speak to your local cider producer and they may help you
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If you plan on using a commercial yeast to ferment your juice then you can certainly add shop bought juice. If you plan on using a natural fermentation with whatever yeast is on the apples then you'd have to be careful about getting juice with no preservatives in it at all.
Obviously don't use too much shop juice as its blended for drinking, not cider making. Up to 1/3rd is probably fine. Sometimes the cheap stuff works better than the fancy organic stuff!