Hire an employee (a consultant)?

Hire is appropriate for a consultant, or more generally for any kind of temporary worker. I’ve also heard “bring in/on a consultant” more than once, which sounds good to me—it conveys more of the “stop in for a few and help out” nature of consulting, I think.

Retain is a good word for this, often used in official contexts. Historically, it has been used to refer to the hiring of legal services. That historical context helps emphasise that a specific service has been paid for (rather than a person employed permanently) and that the term of service may be temporary, with a specific goal in mind.

I'm a consultant, and I tend to be engaged as one by my clients.

In the UK, we might be more inclined to appoint a consultant.