Yoast + manual meta keywords

Hi friends,
I use Yoast on all my client’s WordPress sites. I have a client that is requesting meta keywords be added. I’ve explained in the past why they aren’t really necessary and have spent time refining other aspects of their pages to boost SEO. However, they really would like these terms included- I think it’s a “no stone left un-turned” view. To make them happy, I was considering adding the tag myself into the site header. Before I do that, though, will this cause any conflict with Yoast’s operations? Seems like a silly question, but thought I would ask. I haven’t found anyone with a similar query.
Thanks!