Hello Christoph,
Thank you for reaching out! Yes, it’s absolutely possible to include a button at the end of your menu in Helix Ultimate. You can achieve this quite easily by creating a SP Page Builder module with a button and then publishing it in the appropriate menu position.
For your convenience, I’ve attached a short screen recording in the hidden content to demonstrate the process step-by-step. I hope this helps clarify the implementation!
Should you have any further questions, feel free to ask.
Best regards