CSV import wizard for multiple projects only supports global custom fields

The import wizard disrupts a well working import configuration file when trying to extend the import to more than one project.

Background explanation:

I had to split one of my projects into two projects and wanted to use the same import configuration for both projects.
There are many custom fields in our jira instance, much of them are not configured globally.
The imports are done every day for only one issue type.
Especially the imported issue type has many dedicated custom fields (fields imported from another database).
Exactly speaking: the available context is not global, but restricted to just this issue type.
Sometimes also a project restriction applies, but for the 2 imported projects exactly the same fields are configured.

Now the problem is that it is very easy to change by hand the configuration file so that both projects are imported together with the same import cfg file.
But when I try to make the same changes using the wizard to update the old cfg file, this fails, because all dedicated custom fields cannot be mapped anymore!
The file created by the wizard does not use my custom fields, but tries to create new ones.
It is even impossible to chose the dedicated (=not global) custom fields in the wizard's input mask.
This did never happen when I only had cfg files for importing single projects.
It's only a problem when importing multiple projects with one import cfg file containing the following lines:

existingprojectkey =
importsingleproject = false
importexistingproject = true
mapfromcsv = true

As a jira application admin without direct access to the server, I would like to be able to update the import cfg files using the Wizard.
Otherwise I have to ask every time my IT responsible to upload the updated file.

Using two separate import cfg files for the 2 projects is not an option.
In fact the number of projects concerned by external data imports is increasing and since we are importing each project every day by hand I would like to keep the number of imports as low as possible. Optimal would be to import all projects with one import file.