Created Lead with company name which is already available in the Account. Then I converted the lead to Account and Opportunity. In lead convert page I selected the existing Account, In Account compact layout I added some Standard and custom field. In Lead convert page it shows only the field name not showing the values of the filed as same as for Opportunity also added closed won date but it not showing the existing Opportunity closed won date in Lead convert page.
I recently had same need for the choose existing account results. I wanted to take control of which account fields were going to be shown in the search results when you click on "Choose Existing" field in the account section.
It is controlled by the "Search Layout" of the account object, specific for the current user profile.
Also if the Search Layout allows you to insert more fields, only the first 2 in the list will be shown in the choose existing account search results. The fist MUST be Account Name (you cannot deselect it or move 2nd). The second field is the one you will see immediately after the account name in the search results box (with a lighter fore color).
So I created a custom formula field named "Search Fields" and I used it for my user profile related search result in the second position (after account name).