Hi, i wanted to know if its possible to recycle a bodywork hammer to a war hammer?

Second question would you do it and why?

It depends on what the hammer was made from and how. Using a file to test the hardness at various places may tell you a little about it's construction, spark tests may also help indicate the type(s) of alloy, although I have never been a fan of this method and don't use it myself.
Working with unknown material always involves the risk that the piece may crack or just come apart under heat or the hammer, if you are forging and heat treating. Getting the heat treat right without knowing the alloy can be tricky as well.
If there are markings or identification of some type on the hammer you may be able to find out who it was made by and what the material is, I have contacted tool makers before for this same purpose and had decent luck getting information from them.