structs can contain STRINGs. This is no problem.

As to your copy problem:

struct test{
char charArray[5];
};

STRING mySTR;
struct test testCharStruct;
mySTR = str_create("");

testCharStruct.charArray[0]= 't';
testCharStruct.charArray[1]= 'e';
testCharStruct.charArray[2]= 's';
testCharStruct.charArray[3]= 't';
testCharStruct.charArray[4]= '\0'; /*string termination*/

str_cpy(mySTR, testCharStruct.charArray);

printf("%s", mySTR->chars);

untested. Probably won't work because I did some stupid mistake... but I hope you get the idea.