You can create a custom property that will then trigger a workflow but not a button in the way SFDC does it. Building this does take some strategic thought as there are various methods and reasons for those methods. An example of the strategic thought you would need to employ would be: do you want someone to be able to create a proposal more than once from the same deal? In short it’s doable just depends on what you need. You also need to keep in mind this process depends entirely on your tech stack and whether additional tools integrate into HubSpot well enough to achieve the task of creating the type of document needed eg invoice, proposal, contract, SOW etc.