I don't think it is possible, and I don't think it is a feature that is likely to be added, as it would (potentially) allow you to send an identical email to a unlimited list of people - which is just the kind of behaviour spamgourmet attempts to prevent, if it is unsolicited.
While the vast majority of spamgourmet users are unlikely to abuse such a capability, there are enough anti-social people out there to make the risk of someone abusing it on purpose to give spamgourmet a bad reputation non-zero.
Note this question and answer in the
spamgourmet FAQQ. I just noticed that I can send out messages through spamgourmet that appear to come from my spamgourmet addresses -- can I use this feature to distribute my newsletter to my millions of fans?
A. No, sorry. The sending feature (like the rest of the service) is intended for individual users communicating with single or very few recipients at a frequency that's reasonable for a single user doing that kind of thing. Think: one-to-one, not one-to-many. As an anti-spam service, if we ever get accused of letting our users use our service to spam people (even though you're not spamming anybody, jeez), well, we just can't go there - hope you understand. If you send messages out through the service in a pattern other than the one described above, your account will probably be automatically disabled, and you won't get notified and we won't necessarily get notified and it'll just kind of stay disabled and you'll stop getting any messages. Don't take it personally, that's the way it's gotta be.
I understand it is a bit irritating, as I too have wanted to send an email that appears to be from my spamgourmet address to a small group of people, but I value the continuation of the service more than the small irritation of not being able to do that.