Would it be wise to email their HR person to express interest and ask what skills specifically I should learn for the job?
Yes. and No.
Yes, you can email the HR department expressing your interest for "the" job (specific position / role), but you should not ask about "what skills you should learn". If you're interested in a specific position, that indicates, you know the context of the job and at least an overview of the job profile / description. Required technology / skills are part of that. A "software development job" is too broad description for a specific job.
Instead, you brush up your resume and cover letter, which will show why you are a fit for that role / position / job and let them do the rest.
Otherwise, if you're not sure of the requirement or job profile, simply state that in that communication and be open for any available position they have to offer. This shows you are interested in working with the organization and if they have an opening which matches your skillset and expertise, they will get back to you.
Looking for a specific position and then asking about the required skillset is likely to create a negative impression (example: The inquiry is not serious and based on hearsay).
Disclaimer: If you do not like to read opinion-based response, you should probably stop reading here
Having said that, a bit of personal opinion:
Never use the words "What I should learn / do for the job / position?". Rather use "What are the requirements for XYZ role / position?", or, "What experiences / expertise you are looking for in the applicant for XYZ role?".
While both carries the same meaning, the later questions come across as more positive. At times, the first question may create an impression that you are desperate for the job/position/offer, minus the actual "involvement". Show them you are interested, neither desperate, nor careless.