How to export columns from Advanced Many-to-many object relations

Hi,
I want to know how to export values from column of advanced many-to-many object relation.

This is the relation in the class product

The default grid export only the path

And if I use combination of anygetter operators I cannot display anything in the column.

Can you help me to display in a column stock of export grid the concatenation of qty_available with warehouse code that comes from the object related to the product class?

Something like this:

sku,name,stock
sku1;Item 1;MI1:100|FI1:32|BO1:4
sku2;Item 2;MI1:50|FI1:12|BO1:14
etc…

Thank you.

Best regards