First, you don't need to use a sf::String (it doesn't provide functions for UTF conversions). You have to use the low-level interface, sf::Utf.
To convert to UTF-8:
char utf8[4]; // a single character may be encoded on up to 4 bytes in UTF-8
char* begin = &utf8[0];
char* end = sf::Utf8::Encode(event.TextEntered.Unicode, begin);
someArguments[0] = v8::String::New(utf8, begin - end);
UTF-16 is similar:
uint16_t utf16[2]; // a single character may be encoded on up to 2 16-bits elements in UTF-16
uint16_t* begin = &utf16[0];
uint16_t* end = sf::Utf16::Encode(event.TextEntered.Unicode, begin);
someArguments[0] = v8::String::New(utf16, begin - end);