I have great testimonials from another area of my business — can I use these on my sales page even though the testimonials aren't directly related to the workshop course?

You can definitely use testimonials that are not specific to the workshop course you’re selling.  Just be careful that if someone were to read the testimonial, they wouldn’t assume it’s specifically talking about the results of the workshop course. You would want to make it more general.

So for example, “I’ve worked with Amy in the past and one of the biggest benefits of working with Amy is that she helped me overcome XYZ / figure out XYZ / tackle my biggest challenge of XYZ.”  

The testimonial can speak to your teaching style, your ability to break down content in a way that’s easy to follow, your knowledge, and your experience and expertise as the course creator. It can also speak to your students getting results, as long as it’s clear those results are not from taking the program.