Is it possible to create a drop down menu on a filter?
Yes, make sure your filtered field is set to a category and that the field has its ‘Filtering on this field’ set to ’ A list of all values’
The defaults will depend upon the type and quantity of data, so may need changing.
Ok, i understand.
I’m using an iSeries DB2 database and it’s not possible to define the “Filtering on this field”, because i have no possibility to define the schema in Metabase.
I think, in my case it’s not possible to use that. 
Not sure about DB2 support (it’s currently not in the official releases, but on some fork/Pull Request, right?)
But, otherwise - I think what @AndrewMBaines is talking about is - this https://metabase.com/docs/latest/administration-guide/03-metadata-editing.html (but as I said, not sure what you have the with DB2):
Another option would be to use the linked database feature in MS SQL Server to access DB2 via MS SQL. You’d only need the Express edition. Performance would suffer for complex queries, but might be worth a try.