Is there a way to store int in ObjectID? I'm trying to migrate MySQL user ids to MongoDB. user ids are incremental values like 1,2,3,4...

Is there a way to store int in ObjectID?
If you don't specify _id field when inserting a document in MongoDB, the database internally generates ObjectID and stores the document. In your case, the _id column will be populated with automatically generated ObjectID.

If you want to convert 1, 2, 3, 4 as ObjectID, No you cannot. Because ObjectID should be in 12 bytes . 

The _id Field