This is not possible. The way to change the E-commerce Ext values would be via Mappings. Most tags support overwriting E-Com Ext variables within their Mapping Toolbox. If there is a tag that you are having troubles overwriting via Mapping reach out to your Account Manager and a Solutions Engineer can help with those tags.
So here is our issue: Analytics pixel wants productID to be Foo. Marketing pixels in general want productID to be FooxBar. Would the best way to solve this to make productID FooxBar and have the Analytics pixel ignore the xBar?
Let's assume on your page your setting:
product_id = ["foo"];
I would set the E-com Ext to use product_id as the List of Product IDs option. This will by default tell the tags to use "foo" as the product id.
Then, because this is an array variable, you'll probably need to write some custom code to append "xBar" to "foo" for the product id. Lets again assume that you are outputting this value into a new variable called "marketing_product_id." Then you would map "marketing_product_id" to the product variable in the Marketing Tags. This will overwrite the E-commerce Extension value with the mapped value.
Mock code to append "xBar"
b.marketing_product_id = ;
This will append "xBar" to the product ids.