Trying to transition to ArcGIS Pro and use Portal.
Pro can have 3 types see this: ArcGIS Pro license types—ArcGIS Pro | ArcGIS Desktop
Your question is a little confusing. If you want to use the Portal for ArcGIS component of ArcGIS Enterprise, you can, and you do not have to use ArcGIS Pro.
FYI, ArcGIS Pro has been designed to be able to share content to Portal for ArcGIS - specifically when its been configured with a hosting server.
Configure a hosting server—Portal for ArcGIS (10.5.x) | ArcGIS Enterprise
As Mody Buchbinder's post indicated, you can license ArcGIS Pro in several ways as described in the link he referenced. One of the options is to license ArcGIS Pro via Portal for ArcGIS. Is this what you're asking?
Hope this helps,
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