There isn't a simple method ...

One that I have used in the past is to update all the products_id and categories_id by a higher value than what is used in one database vs the others ...

Then when you import the data there would be no conflict ...

However, you have to ensure that you update all of the fields in all of the tables correctly ...

products
products_description
categories
categories_description
specials
featured
meta_tags_products_description
meta_tags_categories_description
reviews
salemaker_sales

Then if using Attributes ... you need to also update similar issues for products_id as well as the options_id, options_values_id, etc. in all of the attribute related tables ...

It is not a lot of fun to do ... but it can be done if you are careful ...