The widget that displays the button on the product page and in the Quick Look popup is this one:
\XLite\View\Product\Details\Customer\AddButton
You can alter it as follows:
1. Create a custom template in your module (skins/default/en/modules/[YOUR_DEV_ID]/[YOUR_MODULE_ID]/product/add_button/body.tpl) with the following code (it is a slightly tweaked copy of the code from the skins/default/en/product/add_button/body.tpl file):
Code:
<div class="add-button-wrapper {getFingerprint()}">
<widget IF="!isOutOfStock()" class="\XLite\View\Button\Submit" label="{getButtonLabel()}" style="regular-main-button add2cart" />
</div>
2. Decorate the widget as follows:
PHP Code:
<?php
class AddButton extends \XLite\View\Product\Details\Customer\AddButton implements \XLite\Base\IDecorator
{
protected function getButtonLabel()
{
return $this->isSpecialOrderButton() ? 'Special order' : 'Add to cart';
}
protected function isSpecialOrderButton()
{
$product = $this->getProduct();
$result = ...place there your check against the product...
return $result;
}
}
You need also a way to configure which products are regular ones, and which should display the custom button label. Depending on this you should adjust the above text.
As for the popup message: do you mean the message that appears at the top of the page? If so, you should look into decorating the \XLite\Controller\Customer\Cart::doActionAdd() and other methods that it calls.
As you see this may require changing a number of files, so you may be better to hire someone who could code this for you.