Changing a portable basketball hoop to an inground system is the time taking project that you should mostly do on a weekend because it might take 8 to 9mhours easily. All the digging and cement work you will have to do yourself will have to hire a few people as well, for this very purpose.
However, this is not smart to purchase a portable basketball hoop and then convert it into the in-ground basketball system because there will be no more advantages than a portable system offers you.