Creat a Extension of data entity

Creat a Extension of data entity

Add a new field from DataSource of Entity to Fields Section (Just drag and drop)

Right Click Data Entity and Field Corresponding Staging Table. here its CustCustomerV2Staging

Im Extending this to add new field from custTable.

Drag and drop your custom field from CustTable to below Staging table.

Now Build and Sync the Project, Data Entity is good for import and export with new custom field.
Reference

If Custom fields in data entity not populating

1. Generate Mapping for new fields in Standard DataEntity

After Adding new field in Standard DataEntity in above steps-->

Go to DM--> Data Entities --> Search Data Entity --> Click on Modify Target Mappingfrom Action Tab --> Generate mapping

Reference

Refresh Data Entity

In Data Management Framework there are no Data Entities available. But entities are available in Application Explorer in Visual Studio.

Try the following: Open the Data Management workspace and click the Framework parameters tile. Switch to Entity settings and click the Refresh entity listbutton.

https://community.dynamics.com/ax/f/33/t/222835