This should be possible. When you select your R/R pair files in the Portal, you also need to include a '.vrs' (virtual recording session) file. The VRS provides additional instructions including the listen ports and Live Invocation information.
Attached is an abbreviated VRS file with some example content that should help get you started. IMO, it is best to launch Workstation, run the R/R pairs through and save a VRS file at the end rather than attempting to create the instructions manually. Note: If you use WS to create the VRS from R/R pairs, you will need to change the Protocol to HttpProtocolHandler when the VRS is used inside the portal (at least that has been my experience under 9.5.1). You should also remove the R/R Pairs directory that is placed in the XML since you are dragging/dropping the R/R pairs in Portal.
In the attachment look for the Live Invocation information at the bottom:
<?xml version="1.0" ?>
<RecordingSession nonLeaf="WIDE" leaf="LOOSE" asObject="true">
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:
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</Transport>
<!-- This is the VS Listen Port-->
<listenPort>8001</listenPort>
<!-- Live Invocation Target Host Goes Here -->
<targetHost>some.company.com</targetHost>
<targetPort>80</targetPort>
<useGateway>true</useGateway>
<hostHeaderPassThrough>false</hostHeaderPassThrough>
<ssl toClient="false" toServer="false"/>
<basePath>/service/basepath/</basePath>
</Protocol>
</RecordingSession>