Lead to Opp problem | Business process flow

Hi,

Now we have 4 stages in Business process flow - 2 regarding Lead and 2 regarding Opp.

The problem is that user can create lead ( 1. stage ) and click on button qualify lead, and automatically go to 3. sales stage we want to prevent this, because there are some crucial fields ( in 2. phase ) that need to be filed before going from 2. to 3. phase.

So can i it be done, that only after filling all the data in 2. stage user can press the qualify button?

Thanks for sharing your advises and knowledge.