We are using Commerce Server 11.0 in Sitecore 8.1 update 1 and want to upgrade it to Sitecore XC 9.0 update 1. How can I import all the Products from the Commerce Server to Sitecore Commerce engine? Do we have any tool or API for this? What are the steps to import the huge data into the new Commerce Engine?
You can implement your custom plugin and use Commerce Server Api for get products/inventory and create entities for products/categories/catalogs/inventory and relationships in XC9.
For more details, you can follow the Ryan Bailey's instructions: https://blog.ryanbailey.co.nz/2018/05/sitecore-experience-commerce-minion-to.html
Currently there is no out of the box way of doing this. You will have to create your own plugin that reads the commerce server 11.0 catalog xml file and creates the corresponding entities. In the pre-release of SXC9 there was a naive plugin that did something similar, but this only imported the default commerce server fields. In our case, we reverse engineered this plugin and modified it to import our custom fields for development. I can share the source if you would like to. I've hear there are some catalog related changes planned for Update 2, so hopefully it will include an import.
Sitecore Experience Commerce 9 Update 2 now includes a command line migration tool that generates the required Commerce 9 compatible Catalog JSON files from the old Commerce Server Catalog xml files. Have a look at the sample migration plugins inside the SDK.