Using Javascript to add pages to a PDF?

If the pages are predetermined you can use the spawning feature of Acrobat that will work with reader. First,  you will need to add all the pages to the document and go to the 'Organize Pages' Tab. Second, Highlight the first page you want to auto-insert then Go to the 'More' drop down box and select  Page Templates. A box will appear - Name the page and select 'Add'. Uncheck the box next to the newly added page to hide it from view. Do this for every page you want to insert based on field values. to spawn the pages based on inputs create a button to execute the script to spawn the pages based on your criteria.

// Assign Templates to be spawned a variable so they may be recalled;
var a = this.getTemplate("PageA");
var b = this.getTemplate("PageB");
var c = this.getTemplate("PageC");

var resp1 = this.getField("Input").valueAsString;

if (resp1 == "x") {
a.spawn({nPage:this.numPages-1,bRename:false,bOverlay:false});
}
if (resp1 =="y") {
b.spawn({nPage:this.numPages-1,bRename:false,bOverlay:false});
}
if (resp1 == "z" ) {
c.spawn({nPage:this.numPages-1,bRename:false,bOverlay:false});

Yes, this is possible, but it's tricky. Importing data to the file won't trigger it, most likely. You'll probably need to press a button or something to initiate it. Also, it requires installing a script on the local machine, as well as knowing the exact (or relative) path of the files to import.
And of course it can't work in Reader, only in Acrobat.

The basic code to do it would be:

if (this.getField("Text1").valueAsString == "x") this.insertPages(this.numPages-1, "Blue.pdf");

else if (this.getField("Text1").valueAsString == "y") this.insertPages(this.numPages-1, "Red.pdf");