There was some experiment in which a S phase cell was fused with other cell and the other cell also began to replicate DNA. Would the same happen if fused with nerve cell ? Why or why not?
This specific experiment has not been done. Fusing cells is difficult and it also leads to polyploidy. Cells are immortalized by overexpressing oncogenes (or viral replication genes); for example HEK293 cell line was established by transforming embryonic kidney cells with adenovirus.
Now transforming a neuron would immortalize it but would also make it lose its function; for a cell like neuron, replication would compromise its activity for e.g. loss of established synapses. As mentioned in the comments by har-wradim, it was shown that transforming neurons with oncogenes leads to their de-differentiation. Another experiment of transforming neurons gave a contrasting result. Feddersen et al transformed purkinje cells in mice by expressing SV40 viral t-antigen from a purkinje-cell specific promoter and surprisingly it lead to degeneration of cerebellar tissue. Also have a look at this article.