No, if you are already holding an Employment Pass of a company, you cannot serve as a director of another company in Singapore.
Suggested Readings
Can I apply for an Employment Pass prior to incorporating my Singapore company? Why do I need to appoint a resident director?
Can I become a director of other companies while I am on Employment Pass (EP) for one company?
My friend is in Singapore on an Employment Pass (EP). Can he be the local director of my Singapore Company?
Do Singapore Employment Pass holders need to contribute to any social security or skill development fund etc?