Hi nAnirban,
Are you sure you need the conversion exit ALPHA? Usually you find the conversion exit you need under the domain associated to your data. Let's say you have a project definition (just a long shot guess based on your data 
) , so the conversion exits to be used should be CONVERSION_EXIT_KONPD_INPUT/OUTPUT, as it's in the domain PS_PSPNR.
Now, if you don't have a proper conversion exit and simply want to add the zeroes, use ABAP keyword SHIFT.
Cheers,
Custodio