Create Free Membership plan then add existing users to that plan

I know there are options to create a free Membership Pro subscription plan and then when users register on the site they get automatically allocated to that plan. 
Can we create that free plan and then add all our existing registered users (around 3,000+)  who are not members of one of our standards subscription plans to this free plan?
The reason I am asking is that we used Easy Profile Product to collect other information about our users ie Phone numbers. Easy Profile is not supported under Joomla 4. Membership Pro, if we had all our users registered could be adapted to meet our requirements of what Easy Profile provided.
I can't see anywhere in the Membership Pro documentation a way to 'Bulk' add our existing registered users to a plan.
Is this possible?

Hi David

Unfortunately, this is not supported and would require developing a custom script to add all of these registered users to the plan you want.

It will have to add small number of users at a time, not all in one step because it would cause memory limit or max execution time when the script is run

You would need a developer to write this script for you

Regards,

Tuan

Hi Tuan,
Thanks for your reply. Yes, I can understand the issue with lots of users, We have around 5,000 and 750 members.
Kind regards
David