Yes you can, you still have to have a powerful enough military to defend your claim. In fact, given a large and powerful enough military you don't have to buy the land before claiming it.
You'll unfortunately (or fortunately, depending on your point of view) find that military strength costs vastly more than land. Even a billion dollars buys you a ton of land most anywhere, but it buys you vanishingly little security if sophisticated entities want to take it from you.