Question: I was reading about CMS and ERP. And got confused at this point that whether a system can both be an ERP and a CMS? is it possible?

Answer: 
Sure - you can program whatever you like. Whether it makes any sense is a different question.

As for whether having an ERP/CMS hybrid makes sense or already exists - I don't think so. There are some vague similarities and overlaps in that both will typically allow you to define your own entities with fields ("document types" in a CMS, "business objects" in an ERP) and present a web interface for data entry and publishing, but at heart they have completely different purposes.
An example, though maybe a contrived one but one I encounterd in the wild a long time ago:  

An application was built that would store incoming documents pertaining to required reports sent in by customers of a financial services company.  

The same application also served to automatically send out reminder letters to customers who were late sending in those documents, and as an entry point for the telephone support people in the company to look up customer information if customer called (or had to be called).  

In that case both functionalities were closely intertwined. In other systems both functionalities might exist but not be as tightly linked (think something like SAP or Lotus Notes).