Serverless plugin or yaml feature to merge defaults into a serverless file?

I have a serverless file with:

custom:
  specificProp: true

I have another file with a property that should be shared by multiple serverless files:

custom:
  commonProp: true

Is there any Serverless plugin or yaml feature that would allow me to merge the file with the common prop into the first file to produce:

custom:
  specificProp: true
  commonProp: true