String as Foreign Key

I am using the free hosted dsp, and I have two tables (call them XX and YY) where one of them (YY) is updated async by an external system using a string as an ID. At XX I get a hold of this ID when I subscribe to the service, therefore the only relationship between XX & YY is this string ID.
I noticed the FK (type reference) only allows integer, but it would be great for me if when I retrieve XX records I can somehow bring in the same request the related YY records for each XX record.

Unfortunately, scripts are disabled on free hosted app to normalize data further, but I was wondering if there is any suitable in the fantastic range of options that DreamFactory provides when requesting data.