Convert VARCHAR2 to UTF-8 in Oracle 12

Question

I need to convert VARCHAR2 columns that store text in unkown character sets into a standard UTF-8 BLOB.

The table has more columns, but the ones of interest to me are:

ADDRESS     POST
----------  --------------
gibberish1  gibberish2
gibberish3  gibberish4

The "gibberish" is actually good content. It's just I can't determine the specific character set. I assume the engine knows it internally.

If an ideal UTF8() function existed my query could concatenate the whole thing doing:

SELECT RAWTOHEX(UTF8(ADDRESS)) || ',' || RAWTOHEX(UTF8(POST)) FROM TAB1;

Is there any function that can convert to UTF8 without knowing the existing encoding?