You can accomplish this by using javascript.
Please put this script in a javascript file and include that javascript file in pages in which you want to add password protection.
function passWord() {
var testV = 1;
var pass1 = prompt('Please Enter Your Password',' ');
while (testV < 2) {
if (!pass1) history.go(-1);
if (pass1.toLowerCase() == "Password") {
alert('You Got it Right!');
window.open('window.location.href');
break;
}
testV+=1;
var pass1 = prompt('Access Denied - Password Incorrect, Please Try Again.','Password');
}
if (pass1.toLowerCase()!="password" & testV ==3) history.go(-1);
return " ";
}
Edit: Create a module and add a column (yes/no type) in cms_page table using the module.
Show the column in cms page edit form. If it's value is yes for a page, then include the above-mentioned javascript file in the page using layout xml.
Please let me know if it worked for you.