How to merge two .po files?

I have 2 problems for merging 2.po files.

• I have a .po file (trans.po) to localize my applications and another ( transNew.po) that I released recently. I need to combine them. But when I run the following command, I always get the same content as there are transNew.pot, no old translations at all.

  msgmerge --previous trans.po transNew.pot -o transfinal.po

• I manually added some translation string using textedit in the trans.poearlier, and I don’t want to lose them, even if the command can be successful, I will lose this string too. I can not find any option that can solve it.