Yes, providing you have the email beta enabled in your account, you can contact supporters who have registered and created a JustGiving fundraiser via the GivePanel / JustGiving integration.
Please find more information on the Email beta below:
Our email tools work for all registered supporters, including those that have created JustGiving fundraisers.
As well as the standard email triggers and conditions available in the email beta, you will also have the option to segment your registrants by those that are matched to a JustGiving fundraiser, who have either claimed or not claimed their JustGiving fundraising page.
You can also include the short code [UnclaimedPageURL / ] in these emails to generate claim links for supporters who started fundraisers, but haven't claimed their page yet.
Click for more information on our GivePanel Email Beta