Of course you can hold both licences. Note, Australia doesn't recognise the term "frozen" ATPL. It's a marketing short hand term. In Oz it's known as a CPL/IR with ATPL credits. In other words, your licence is a CPL until such time as you have the hours to get an ATPL.
You can search for jobs with both... However, it's very hard to get a job if you aren't in the area. I don't know how much you know about the Australian aviation industry but it is extremely rare for a non cadet to get a job with Qantas (for example) with less than 500 hours in command. Most people start their aviation career in Oz by either instructing or bush flying.
It is also a costly conversion to the JAR ATPL as well. If you really want to do both, go for it, however, it isn't a straight forward conversion.
PPL's are comparatively easy to change over. CPL's + are not.