Hi,
There is no pre-made system to add a fixed amount to the price of the product based on whether something has been entered in a custom item field or not.
However, we do provide a "donation" plugin which can take the value entered in a custom field and replace the product price with it:
demo.hikashop.com/index.php/en/hikashop/...t-page/donation-page
So it would require modifying the code of that plugin.
You would have to change all the instances of:
hikashop_toFloat($product->$column)
to:
hikashop_toFloat($product->prices[$k]->price_value + XX)
where XX is the amount to add to the product. That change is to be done in plugins/system/custom_price/custom_price.php
Then the donation plugin will behave like you need it to.