How can add all member of a Dynamic Distribution group to a new Security Group

You can view all the members this way:
https://video2.skills-academy.com/en-us/exchange/recipients/dynamic-distribution-groups/view-dynamic-distribution-group-members?view=exchserver-2019

Example:

$group = "Full Time Employees"  
Get-Recipient -RecipientPreviewFilter (Get-DynamicDistributionGroup $group).RecipientFilter  

Now that you have the group - you can output the SAMAccountName

$Users = (Get-Recipient -RecipientPreviewFilter (Get-DynamicDistributionGroup $group).RecipientFilter).SamAccountName   

$users should show all the SAMAccountNames of the group members Use that to add to the new group ( I am assuming this is an on-prem AD group)

Use: Add-ADGroupMember

$users | % {Add-ADGroupMember -identity SecurityGroup -members $_ }  

https://video2.skills-academy.com/en-us/powershell/module/addsadministration/add-adgroupmember?view=win10-ps