Can research be conducted from a holding that hasn't been converted from Ex-Ante Research?

Yes, you are able to select a new holding (including generic) and create an ex-post review.

When the ex-post review loads, you will see the same screen as when using a converted holding from ex-ante research, but there will be no Previous state. 

As seen below:

For more information on how to create an ex-post review, see articles:

Video - Ex-Post Reviews.

Article - Ex-Post Reviews - How to create a review