What you're asking for is copying the field "Account Manager" in entity "Account" to entity "Opportunity", is it?
If that is what you want, you need to create a field named "Account Manager" (or whatever you wish to) in the "Opportunity" entity form; the field type being the same. And you need to have some kind of workflow/plugin/javascript to update the field in "Opportunity" form with the same value as present in the "Account" form.
EDIT: Based on your comment, I've edited the answer. You can achieve it by two means:
- Have some kind of workflow/plugin populating the field in Opportunity form from that of the Account form.
- Fetch the value of the Account field by javascript on Opportunity form load and display it in a field.
Also, you might want to make the field read-only in Opportunity form.