CSV Import Product Update

[DJPhill]

Hello

Running on PS 1.6.1.18 - not a problem with formatting and importing a CSV correctly.
However; I need to update some products into additional categories but when I import the CSV, the new categories replace the old ones. It doesn't just add the product into the new categories and keep them in the old categories.

Is there tick box I'm missing or an option to update rather than replace?

Alternatively does anyone know a way of exporting all products with categories as ID numbers instead of names?
I have seen this http://nemops.com/prestashop-export-products/#.XnzAfur7Tcs
And this 

 

But both options export categories as text rather than ID numbers.

Many thanks.