Something

char* insert_char(char *s, char c, unsigned int pos) {
    int i;
    int s_len = strlen(s);

    if (pos <= s_len) {
        char *new_s = malloc(sizeof(char) * (s_len + 2));
        new_s[s_len] = '\0';

        for (i = 0; i < pos && i < s_len; i++) {
            new_s[i] = s[i];
        }
        new_s[pos] = c;
        for (i = pos; i < s_len; i++) {
            new_s[i+1] = s[i];
        }
        free(s);
        s = new_s;
    }

    return s;
}