Please read below the reasons why I

A) will not natively support this option (i.e via a checkbox or something) and
B) how to get around this if you must

A) First of all , this is of course just a personal opinion and you might entirely disagree, which is fine with me. However, if you were considering making registrations mandatory, you might want to consider the following first:

Putting myself in the shoes of a customer (and obviously having been one myself on various sites) I really fail to see , why I *must* register to just buy a product from a(ny) shop.

In fact, many times I have been to sites that wanted me to do so and forcing me to enter all sorts of – quite frankly irrelevant – personal information that had absolutely nothing to do with the purchase I wanted to make.

I’m sure I’m not the only one, but this simply resulted in taking my business elsewhere, never to return.

Entering my personal information should be of benefit to me – and therefore voluntary – and not to the shop so it can harvest and use my data as it sees fit.

Sure, if it suits me to have my personal details filled in if I suspect I will return to the shop at some other times get some discounts in the future perhaps or things along those lines, I happily register as it will make my life easier or perhaps save me some money next time.
If however this is a one time only purchase (for the time being at least) I most certainly don’t want my info to be used (and possibly abused being spammed at etc.)

Again, you might disagree and I have no problem with that. Just thought I’d write a bit why I personally think this is just not ever a good idea. (in fact, as far as I’m concerned it’s right up there with blinking text, gratuitous popups and autoplay of music etc…)

B) If you still feel you must force the user to register before he/she can order, you can either write some javascript that pre-selects the appropriate value (i.e “continue as” -> “create account”)
and hide the whole thing via css (or something similar)
or
use some other registration plugin and conditional statements (plugin, shortcode or otherwise) that will display a registration/login form on the order page instead of the order itself if the user is not logged in or has not yet registered

if your chosen registration form plugin does not automatically add any form fields that you have enabled in the WPPizza->order from settings-> “use when registering” to the registration process, then it’s creating its own registration page rather than using the appropriate WordPress filter hooks. Use another one.

Anyway, that’s it from me regarding this subject…

Thanks for reading

PS: some more conversation on this topic
https://www.wp-pizza.com/topic/a-comment-about-registrations/