I want to customize my standard conversion page by URL hacking I am able to check the opportunity checkbox but can't find this issue .

Requirement :
 
My requirement is that I have accounts present in system and the lead is always converted with existing account. I don't want to allow user to create new account in salesforce so there is no need of that create new picklist value.

 There is a picklist of Account in lead conversion page what I exactly want is :

1) I want to show only one picklist value that is 'Attach to Existing: BigLife Inc.'
2) Ability to associate it with that account.
2) Make that field read-only so that user can't able to change it.

Below is my script

window.location.href="/lead/leadconvert.jsp?nooppti=1&id={!Lead.Id}&RetURL=/{!Lead.Id}";