Hi,
I assume that you refer to the default defined in voucher type per usergroup. 
I see no way to define it there and use it in external files. 
When importing external vouchers the parameter set is used defined as default at the File Type connected from the Parameter Set Id to the Load type:

In the standard parameter set the load type (load type connects to voucher type in a specific company) is defined to be used for fetching the default value.

I can now imagine the following setup to be supportive (keep in mind that the file types are relevant for all companies).

You can add additional parameter sets and set another set as default. I explain 2 Solutions (You need to copy the details from Set1).

1.    Fixed Load Type
Set the load type Fix to the type you would like to have:

 During import this Load Type will be always the default and/or you can change it manually:
 

 2.    You can use a different Variable like User or User Group

You need to have a Load Id for each USER or USERGROUP defined.
 

How it looks during Import:
 

Of course there is some overhead in these solutions and other imports (for external vouchers using the same file type) need to be considered as well.
Regards
Ralph

In Addition:

You might change the Validate Method (use another api.call) to fetch e.g. the default voucher type for the user group the user belongs to as a synonym to the load type.

Thanks Ralph - This will work.  I can assign a specific parameter to my users that need a different default.  I appreciate all the screen shots.