Overall, you should work to remove this from your sales and marketing process. It is against best practice to have leads in either system without email addresses.
That being said, (and it pains me to say this), you can upload those leads without email addresses as X@x.com , or some dummy email address.
In all your marketing activities, suppress that email address, such as Send email except unless contains X@X.com. You can also suppress scoring activities, reporting, everything by saying Email Address not contains X@X.com.