Add existing sites to Pro Sites, no Activation key

I am converting a very old website into a WordPress Multisite system. The old system was a subscription-based site, so I plan to implement Pro Sites to maintain that functionality on the new site. But of course, I have to migrate old sites and their subscriptions to this new site.

The ripple I've encountered is: because these sites have been created dynamically by batch, there is no wp_signup record for any of them. Looking through the activate_user_blog() function of Pro Sites, I see that it wants to grab that activation key to verify that the site is legitimate, I presume.

But in my case, there is no such record. So I'm unsure of how to proceed. I'm inclined to want to stick to the API for activating users, since it obviously does a lot of work. But without this key, I'll be unable to continue.

One idea I had was to simply extend the ProSites class and rewrite that function without the check. But that seems like a dubious prospect.

Another answer I've though of is creating records in the database for all the sites I want to activate and just put garbage in the activation_key field.

Does any of this make sense, or am I reading the code wrong?