Looks like you want something like
#include <string.h>
...
/* sizeof data is 64 if data is an array[64] of (unsigned) char */
memcpy (&buffer[6], data, sizeof data);
This copies all the 64 data octets to the 7th buffer byte and following. If you want to copy less data,
memcpy (&buffer[6], &data[42], 3);
copies the three bytes starting with data[42]
.