Yes, this is appropriate if you are confident that neither you nor any other expert could confidently identify it to a single species.
Observations fields are a free-for-all. Technically anyone can create one for any purpose you want (as long as it’s not inappropriate). So if you think there is a need for a certain holding bin (it doesn’t already exist) and would be useful for you and others, go ahead. If you’re working with spiders for example, it may be better to communicate with arachnojoe and get the option added to his observation field which already exists for that purpose.