No, that is to misunderstand the point Paul is making in Romans 2:28 (in Paul's SECTION covering "Israel" [and in a DIFFERENT section is covering "the Gentiles" and then has his CONCLUSION in chpt 3 that NO ONE is righteous...])...
so in the SECTION re: ISRAEL, Paul in Rom2:28 is speaking WHOLLY and ONLY of "JEWS"... "he is a Jew, which IS ONE *inwardly*" (this is NOT covering "the Gentiles" here AT ALL [that is a separate SECTION, where he covers those]. IOW, in this verse, Paul is not saying "a saved GENTILE *BECOMES* a Jew!)