Update Meta Description + Keywords

Hello,

I would like to update meta description and keywords, keeping the current words.
I suppose that I have to use Concat or am I mistaken?
And if I have to, what would be the exact expressions? Like: Concat( product_meta_description, ",my_new_isbn");

Thank you in advance.

Hello,

Ok, I will do that.

Thank you nicolas.

Hello,

And if I would like to update the value from a custom field's content, would be like:

concat( product.product_meta_description, ',' , product.custom_field_name) ?