零的数量需要根据剩余字符串的长度动态生成，所以我认为在本机pig中不可能。
这在自定义项中是非常可能的。
输入文件

11111
222222222
33
org.apache.hadoop.util.NativeCodeLoader
apachepig

Pig手稿：

REGISTER leftformat.jar;

A = LOAD 'input.txt' USING PigStorage() AS(f1:chararray);
B = FOREACH A GENERATE format.LEFTPAD(f1);
DUMP B;

输出：

(0000000000000000000000000000000000011111)
(0000000000000000000000000000000222222222)
(0000000000000000000000000000000000000033)
(0org.apache.hadoop.util.NativeCodeLoader)
(0000000000000000000000000000000apachepig)

udf代码：下面的java类文件编译并生成为leftformat.jar
左键盘.java

package format;
import java.io.IOException;
import org.apache.commons.lang.StringUtils;
import org.apache.pig.EvalFunc;
import org.apache.pig.data.Tuple;

public class LEFTPAD extends EvalFunc<String> {
@Override
public String exec(Tuple arg) throws IOException {
       try
        {
            String input = (String)arg.get(0);
            return StringUtils.leftPad(input, 40, "0");
        }
        catch(Exception e)
        {
            throw new IOException("Caught exception while processing the input row ", e);
        }
    }
}

更新：

1.Download 4 jar files from the below link(apache-commons-lang.jar,piggybank.jar, pig-0.11.0.jar and hadoop-common-2.6.0-cdh5.4.5)
http://www.java2s.com/Code/Jar/a/Downloadapachecommonslangjar.htm
http://www.java2s.com/Code/Jar/p/Downloadpiggybankjar.htm
http://www.java2s.com/Code/Jar/p/Downloadpig0110jar.htm

2. Set all the 3 jar files to your class path
  >> export CLASSPATH=/tmp/pig-0.11.1.jar:/tmp/piggybank.jar:/tmp/apache-commons-lang.jar

3. Create directory name format 
    >>mkdir format

4. Compile your LEFTPAD.java and make sure all the three jars are included in the class path otherwise compilation issue will come
    >>javac LEFTPAD.java

5. Move the class file to format folder
    >>mv  LEFTPAD.class format

6. Create jar file name leftformat.jar
    >>jar -cf leftformat.jar format/

7. jar file will be created, include into your pig script

Example from command line:
$ mkdir format
$ javac LEFTPAD.java 
$ mv LEFTPAD.class format/
$ jar -cf leftformat.jar format/
$ ls
LEFTPAD.java    format      input.txt   leftformat.jar  script.pig