I have an a webpage (not associated with Marketo) that has a link that when clicked, opens a form. This is a form from our old marketing program. I was wondering if it possible to change the link so it opens a Marketo form.
Yes, this should be possible. It depends a bit on how you want it to work.
If you use the Form's Embed Lightbox code, you can have this form pop up as a lightbox. If you want it on a separate page somehow, then you need to createa new page, add the Form, and then change the link on the first page. This is well documented on docs.marketo.com.
There are some threads on adjusting the behavior for this. Please do a search.