There is a Sharepoint connector that works for some simple migrations that can be enough so start by checking out that. You can trigger a manual update between the two systems to transfer documents and site pages etc.
If you need a more advanced one time migration I would build a new import class on the Sharepoint side that uses Episervers rest api to create content and media. If you haven't got the possibility for using the rest service I would export Sharepoint content into xml and then build a custom scheduled job on Episerver side to import them. Hope that gives you some working option.
Check out service api for creating content in Episerver here
Anyone had experience migrating SharePoint pages to EpiServer pages? Just curious what software / tools you used to achieve this. Thanks for your time!