if your Infinia add-ons have same card number as primary card (when the add-ons are sourced offline), I assume you can swipe them as well..
I had swiped add-on card once, but wasn't charged. I did not take such risk after that and always request add-on members to use PP
The thing is, we don't know when HDFC would devaluate Infinia and would stop domestic access via PP and so, using CC seems safer (eventhough they say that only primary can be used)