Venu, Please find below the script for your current requirement. I hope this helps.
if(nature = 'assistance' or nature = 'correction', 'incident',
if(nature = 'evolution', 'changemgmt',
if(nature='project','project'))) AS ticket_type
If you are going to have multiple values, then form an inline table with two fields nature, ticket_type and use applymap in the load script. (or) You can join the table with ticket_type with your TableXYZ and still get the ticket_type field in your table.