Splunk Extracted Fields work on existing data they do not offer expression evaluations. If you want to move your above string concatenation to Knowledge Objects so that you don't have to write the extraction every time during search you should consider moving this to Settings --> Knowledge --> Calculated fields, where you can write any valid evaluation expression.
You would need to provide other details like your Destination Splunk app name, Sourcetype to apply eval expression, name of eval field and the eval expression. In your case
Name= fields
Eval expression= subrackNo."-".SlotNo."-".boardType
Once you do the above fields field should be available under Interesting Fields, when the search is run in Smart or Verbose mode.
PS: You should also make sure that you push only those eval expressions to Calculated Fields which are universal to your sourcetype, like your data contains bytes field and all your report/dashboards show kilobyte, then you would want to move the eval expression to Calculated fields i.e. eval kilobyte=round(bytes/1024,0).
In your case, if not all your dashboards use above string concatenation there is no point evaluating the same upfront through Calculated fields, when they are not going to be used in all of your searches. In other words, through Calculated fields, you pipe an additional eval expression to your existing base search. If left unused this will be an overhead in the SPL. So please think it over before creating a Calculated fields knowledge object.
Also another thing to consider is the precedence of Calculated Fields
1) search-time field extraction
2) field aliasing
3) Calculated Fields
4) derivation of lookup fields.
So, what it implies is that, Calculated fields can not be used on your lookup table fields. Refer to Splunk documentation on Calculated Fields: http://docs.splunk.com/Documentation/Splunk/6.5.1/Knowledge/definecalcfields
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