It is now possible to export aggregatable custom metrics with Funnel's data warehouse connectors to Google BigQuery and Amazon S3. For a metric to be aggregatable it must be possible to sum its values to a meaningful total.
For example, if you create a custom metric to combine all conversions measured by your various advertising platforms (e.g., AdWords conversions + Bing Conversions + Facebook conversions), the value for one day plus the value for another day equals the total value for both days.
An example of a non-aggregatable custom metric is Cost per Session (Cost / Sessions). The value for one day plus the value for another day does not necessarily equal the total value for both days. To tell the total value you need to look at Cost from both days and divide it by the Sessions for both days.
When setting up your export, metrics that are aggregatable can be selected in the field selector, while non-aggregatable metrics can not: