After you temporarily change the format to Historic, you can select non-Standard sets from the Advanced Filters screen. You can then add/remove cards from your deck. When done, you can change the format back to Explorer, and any cards that are not valid in Explorer are dimmed.
Adding to turniptruck's workaround:
After you temporarily change the format to Historic, you can select non-Standard sets from the Advanced Filters screen. You can then add/remove cards from your deck. When done, you can change the format back to Explorer, and any cards that are not valid in Explorer are dimmed.