I want to pull data from another data source using certain fields in my existing data set and where the match does not exist I'm fine with getting NULL value. Something like GETPIVOTDATA in the excel which we use to pull data for matching fields.
I'm not sure if this is what you were trying to achieve, but I have a sample workflow attached.
First text input has some field names... second input is your "other data source" that shares some of the same field names. Basically you bring in the whole data source and then through a series of tools (Union, Transpose, Filter & Cross-tab), you're able to filter the second data source to only the specified columns. It doesn't feel very efficient (would love to see if someone else has a cleaner solution)... but it should work for any number of chosen fields.
(Might be "cleaner" looking if this does work to turn it into a macro, so that your actual workflow shows just the one "tool" instead of the whole workflow.)
If that's not quite what you were looking for, could you perhaps attach sample data showing what you start with and what your expected outcome would be? Thanks!