The instructions for assembling the Cimarron 55x12x12 batting cage are available in the Owner's Manual tab of the product page. You can decide how easy to assemble it is for yourself.
If you wanted to extend the cage another 15 feet beyond the 55 foot cage, you would need to purchase the additional extension poles, hooks and elbows directly from Cimarron Sports (they sell everything from complete cages to just single pieces of them).
You could either buy a custom net directly from Cimarron that you would need to cut the end off of and stitch onto your current 55 ft. net to get it to 70 feet or you could just order a whole new 70 foot net.
Although the price of a 70 foot Cimarron batting cage net is available on our website, that price is different depending on the net gauge you choose. We do not know what individual batting cage parts or a custom net size would cost. Cimarron does not even show those things on their site but you can contact them directly and they will tell you.