Thank you for your post.
Yes, you can create a Conditional Value List in a portal.
A Knowledge Base article (#5833) guides you through creating a Conditional Value List. You can view the article at:
Just make sure you apply the changes to the fields in the portal. If you are still having difficulty, then let me know what fields and values you want to display in the portal, and I'll give you the exacts steps.
Keep in mind that when you set up a conditional value list the "table context" for your conditional list changes when you format a portal field with the value list instead of a field on the main layout.
Think of the portal as a "mini-layout" with the portal's specified "Table Occurence" as it's table in similar fashion to how your layout refers to a "Table Occurence" in its Layout Setup. That difference can affect what values you see appearing in your conditional value list.