Per your description, you want to use a column as the approver parameter.
You do as the followings:
1. Create a site column using People or Group column, create a custom Document content type, add the new site column into the custom Document content type
2. Create a library and enable "Allow management of content types" in Advanced Settings, add the custom Document content type into the library.
3. Open the site with SharePoint Designer 2013, go to Workflows tab
4. Right-click "Approval-SharePoint 2010" and click "Copy and Modify"
5. Type a name for the workflow and select the custom Document type in Content type:
Click as the following:
Select the people column created in Step1, as the below:
After the above, publish the workflow and create a workflow for the custom content type based on the new reusable workflow in browser.
The approvers will be from the people or group column of the custom content type when you use the custom content type for your documents.