With a proviso - you will want to keep the gain way down to avoid overloading the radio. You would need to make a cable with a 3.5mm plug for the Grundig, and likely a PL-259 on the other side of the cable.
It's an awfully expensive solution, though. A small amplified loop such as the Kaito and a few others would be a much more effective solution. Not nearly that expensive, either. Our loop antennas wiki has a link for the Kaito - go to the top blue toolbar, click on 'wiki', go to 'Forum Specific Topics' then 'antennas' - you will find a link that says 'loops'. There's also a link to a review of the Degen vs. the venerable Sony ANLP1 a bit further down the page
Loops have an advantage in that they are less sensitive to local noise sources than a wire or vertical. It's something to consider if you are restricted to indoor antennas. Of course,if you like to build something, I highly recommend the Carpet Loop. Very simple construction and a great place to start for a newcomer - and you'll learn a little about loops in the process (a definite win-win situation if ever I saw one....).
73s Mike