I am trying to work out if I can use a conditional statement on a merge field that is supplied by a zapier integration call.

It appears as though I can only add and edit conditional statements on fields that are listed as being supplied by a datasource. I am currently supplying fields via a zapier integration and therefore when I add a condition nothing is listed in the dropdown selection to use for the condition!!  Can someone tell me if there is a workaround to this?