If these would be specific, predetermined values you expect the buyer to enter in the cart manually and accurately, you might be able to use our Discount Codes feature for that:
Basically, you would create a Cart Discount code in advance for every value you expect buyers to enter, although you could define these to grant 0.00 actual discount off the order. Then whenever a buyer enters a code in the cart's Discount Code field (you can change the field's label to say something else), if that happens to match a Cart Discount code you've already defined, then that value would be passed with the order data.
Otherwise, could add a &custom=YOUR-VALUE-HERE parameter to every Add to Cart button's URL. This could be a static value hard-coded into the buttons on each page, or you could have some custom scripting in your page that appends an arbitrary custom value to every button dynamically. This would ensure that the custom value gets passed into the cart regardless of which product(s) are being ordered. Only one custom= value can get passed per order, and the last item with a custom= value that gets added to the cart determines the custom= value that gets passed with the order.
Was there a particular purpose you had in mind for this? If you could describe what you're trying to accomplish, we might already have a more straightforward way to meet your need.