Joining multiple fields to create new meta field

Hi Team

We want to add some extra meta fields to the indexed data with value created by joining the value of two or more existing meta fields or by manipulating the value of existing meta field.
ie if we have field1 & field2 already in the repository data.
We want to add field3 with value "some contant string"+ value of field1 + value of field2
Regards
Hemant