We vote for delegates in PA.
There is a selection process for delegates in each state. If you want loyal delegates, you need to have people in the party in each state who actually like you. That's the problem with trying a hostile takeover of a party.
We come to the difference between the "party organization" (ie, the party Establishment), and the voters who are registered with that party, and what happens when their interests diverge.
It's like when you have a corporation which has a management team which is working for their own interests, rather than the interests of the stockholders. The only solution in that case is for the stockholders to allow a hostile takeover (hostile to the interests of the entrenched managers), so that the managers can be purged, and new management installed who will further the interests of the real owners of the organization.
So, you're right, what Trump is trying to do IS a hostile takeover. A takeover of the party establishment, to replace them with people who will actually respond to the desires of the voters.