My guess would be to build an App that is the base data model to test your data quality and build your master item list. Not sure you can import master items programmatically yet but would be a nice feature.
I guess we have to copy them in the old fashioned way
Until we can use Master Items directly in our expressions they are a bit of an overhead to be honest except when managing common visualisations e.g. Calendars that can appear on every sheet.
You can import master items, measures and dimensions using the APIs.
I manage my measures in a excel spreadsheet for example and just import them using the APIs.
It's fairly custom as I also specify my main fact field to generate the standard date calculations, YTD/MTD etc etc
But you could most likely simplify quite a bit if you just want to share the definition across apps.
One old fashioned way that IS dynamic and was considered a best practice is to use variables to store the expressions in a spreadsheet for example.
You can blend this approach with master items by using the variable expression as the DEFINITION of the master item
Step1: Write out your expression in an excel spreadhsheet using syntactically correct Qlik Sense expression
Step2: load the expression into ONE OR MORE Qlik Sense apps using the load editor
in this example, i load only the 'Regions' master item from the spreadsheet. The expression is stored in the variable 'vRegions'
Step3: Create a new master item with the same name as the loaded expression.
Now, if ever you want to change the defintion of the 'Regions' master item. Just change the expression in the spreadsheet and reload all the apps ... one place to change everything. Works for Dimensions and Measures