This is an idea, every other ATS system I have used (Greenhouse, et al, even ADP) allows internal users to add an applicant when they receive a resume via email. This system needs this functionality. It also needs to allow us to have a talent portal where we can invite people to apply to positions or even add them to the position.
If you want to add the applicant with the resume and add them to the application pool immediately, then you can submit your request as an Enhancement request. You can log this as an Enhancement Request on PowerSchool Ideas Portal so that the product team can take a look at this. This article should walk you through how to raise an Enhancement Request.
However, if you want to add them to the system and invite the person to fill out an actual application, there is a method for doing this using Job Fairs. With Job Fairs, you can create an event (you could have a standing one for email resumes you receive). From there you can enter in the candidate's information, send them an offer to apply, and also see if they did apply or remind them if they haven't applied yet.
Please let us know in case of any further queries.