On the question of the convert of leads

Hi all,

      When i wanna convert a lead to sales lead , i find a question that if the lead was created directly, then at the actions option, we can see 'convert to sales lead' and other options. Look at the screenshot below:

     But if a lead was created by 'Use Existing Account', we cann't convert it to a sales lead, here at the actions option, only display 'convert to opportunity'.

How can i to convert to sales lead by Use Existing Account.?