As far as my research has taken me, I cannot see a way to convert a Dynamic group to a static one in office 365. To make the process easier, we can use PowerShell to duplicate the group then delete the old one.
$DynGroup = Get-DynamicDistributionGroup "All Staff"
$DynGroupMembers = Get-Recipient -RecipientPreviewFilter $DynGroup.RecipientFilter
New-DistributionGroup "All Staff"
$DynGroupMembers | foreach {Add-DistributionGroupMember "All Staff" –Member $_.Name}