Workflow in the existing Apps

I know we can create workflows for the new app. But, is there way to add workflow in the existing apps such as Sales or Purchase, like “Send for Approval” -> “Approved”.

Secondly, can Workflow be dependent on value of a field such as if Amount is > 100 then “Send for Approval” etc.

Third, any action can be trigged via workflow such as if “Send for approval” then run following custom code.