Can I transform already normalized rna-seq data with Z-score? I have HOMER diff rna-seq data with normalized and want to draw heatmap with z-score. Maybe question might be dumb but please enlighten me.

Depends what you mean by 'already normalized RNA-Seq data'. If you have logCPM values and want to have a heatmap that isn't dominated by the expression level of each gene, then it's common to convert the logCPM values to Z-scores.

As an aside, your question is off-topic for this site. This site is meant for people with questions about Bioconductor tools. Regular basic questions like yours should be asked on biostars.org instead.