Shemona Esrei

Question

Kavod HaRav- I learned that the Sh'lah says that when davening Shemona Esrei, one may- and should- include his own personal addition, and not just in rifa'ainu and shema kolainu. For example, when saying v'lamalshinim, one may insert before the end of the bracha his own tefila relating to the bracha in the siddur, like "HaShem please destroy our enemies, specifically ____.....". Is this truly permissible, and not only permissible, but preferable??? Thank you!