public String Convert15CharTo18CharId(String id) {
    String suffix = '';
    Integer flags;

    for (integer i = 0; i < 3; i++) {
        flags = 0;
        for (Integer j = 0; j < 5; j++) {
            String c = id.substring(i * 5 + j, i * 5 + j + 1);
            //Only add to flags if c is an uppercase letter:
            if (c.toUpperCase().equals(c) && c >= 'A' && c <= 'Z') {
                flags = flags + (1 << j);
            }
        }
        if (flags <= 25) {
            suffix = suffix + 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.substring(flags, flags + 1);
        } else {
            suffix += '012345'.substring(flags - 26, flags - 25);
        }
    }
    String convertId = id + suffix;
    //18 Digit Id with checksum
    System.debug('convertId-' + convertId);
    return 18 digitId;
}

Example:

String Id = '0019000001EJNfj';
String convertedId = Convert15CharTo18CharId(Id);