How to concatenate an hex to a string in C

I'm trying to create a string that's composed with multiple parts. It starts as a normal string and at some point a function is called that fills a pointer with an hex number. Like shown below.

PVOID hexval = NULL;
PCTSTR mystring = TEXT("BLA");

function(&hexval);

mystring += hexval;  // just an idea of what I want, not actual code

As shown above I want mystring to have the hexvalue appended to it. Assume my hexvalue is 0x424C41. I want to end with mystring being "BLABLA".

What's the best way to do this?

#include <stddef.h>
#include <stdlib.h>

#include <windows.h>
#include <tchar.h>

int main(void)
{
    ULONG_PTR value  = 0x424C41;
    LPVOID hexval    = &value;
    PCTSTR mystring  = _T("BLA");

    size_t length    = _tcslen(mystring);
    size_t new_size  = length + sizeof(ULONG_PTR) + 1;

    LPTSTR new_string = calloc(new_size, sizeof(*new_string));
    _tcscpy(new_string, mystring);

    size_t offset;
    for (offset = sizeof(ULONG_PTR); offset && !((char*)hexval)[offset - 1]; --offset);

    for (size_t i = length ? length : 0, k = offset; k; ++i, --k)
        new_string[i] = ((char*)hexval)[k - 1];

    _tprintf(_T("\"%s\"\n"), new_string);
    free(new_string);
}

Maintaining ANSI-support in 2019 is masochism, though.