Making Account field on opportunity mandatory based on sales stages

 Hi,

Is it possible to have the ACCOUNT field on opportunity layout mandatory based  on different sales stages. e.g. We don't want it to be mandatory at the earlier sales stages but required at later stages. We normally control other fields using the sales process, but the problem is that I cant find the field "Account" in the sales process where you can create additional required fields. 

Any suggestions on how to achieve this..

Thanks,