Yes, this can be done, but there is a danger in here. Currently Zen Cart checks whether a product can be placed directly in the cart and if it can displays an add-to-cart button. If it can't, it displays more info instead.
If you put in an add to cart button for all products then you risk products being placed in the cart without, for example, knowing the values of required attributes.
So if Zen Cart is displaying more info buttons instead of add-to-carts to force users to the product info page to choose attributes, it would be dangerous to over-ride it.
If on the other hand you want to add more info buttons to products that would otherwise display add-to-cart buttons, that would be safe, and you would find the code needed for that already in the add-to-cart section of the listing template.
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(Questions answered in the forum only - so that any forum member can benefit - not by personal message)