On click of "Merge Accounts" action link from account page it will take us to merge/accmergewizard.jsp wizard where we can Merge Accounts by Finding Accounts. On click of Find Accounts button you will get a list of accounts with fields called Customer Portal, Partner Portal, Account, Site and Owner. Now my question is how can we add few more fields into this list (Merge my account screen)?
I am afraid that this feature is not yet available on Classic UI, there is an idea which is quite under threshold votes: Customize Columns on Step One of Merge Accounts (CLASSIC).
Good news is that this feature is available on Lightning.