I’ve done this for my site. It’s ugly, but it works.
Go to MailPoet in your WordPress and create a new signup form via the Settings tab. Then at the bottom of the page there’s a “HTML” link where you can see the HTML to show this form. Copy that HTML and paste it into your SumoMe in the “Advanced” tab for the ListBuilder or ScrollBox widgets.
It works, but SumoMe won’t report subscribe completions, and you’re limited to the styling in the MailPoet form builder unless you’re a CSS hacker.