Yes, you can migrate you Commerce Server Catalog to XC9.
First of all, you need to extend Catalog system entities schema link
Detailed instruction you can find here
After that, you can implement your custom plugin in XC9 and and call there Commerce Server Service API for get catalog structure/data. Use XC9 Engine API for create the same catalog in SC9.
Look at Postman Collections (Sitecore.Commerce.Engine.SDK), which Sitecore team provides OOTB. You can find there all examples of requests to SC Engine.
When your catalog will be created in XC9, you can use Postman request for export you catalogs. Also you can create your custom environment and add there some pipelines which will create catalog base on your exported catalog during
InitializeEnvironment your custom environment.
As an example, you can look at