Yes you can do that but it’s a long journey. I’d recommend spending a night in Hamburg or on the Hamburg - Copenhagen night train. It arrives quite early in the morning though (only one pass day used as the departure day counts).
On a pass day you can take as many trains as you want. On 2 of those pass days you can travel in your own country.
Remember that the Eurostar requires a mandatory reservation : 30€ to Paris, Lille, Brussels and 35€ to Rotterdam, Amsterdam. It’s a popular route and there’s a passholder quota so book well in advance. What would be your travel date ? Check availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish
As a good alternative there’s the daytime or overnight Harwich - Hoek van Holland (Rotterdam) ferry : 30% discount with Interrail.
The vast majority of trains can be booked online. Exception : some Spanish regional trains and trains in Romania, Bulgaria, Turkey.
Questions welcome. :)