Providing a research assistant post isn't for people who already have a PhD there is a chance that you would be able to register for a PhD. If you see an ad for a research assistant, write to the supervisor and ask if the successful applicant would be able to register for a PhD. The problem is that the body funding the research assistants’ salary is paying for a full time research assistant, not somebody who is working towards a PhD. They can stipulate that any work towards the PhD is done in addition to the 9-5 they are paying for. How you juggle or fudge these restrictions is between you, your supervisor and your university.
There is no secret to getting these posts. Like most things you just have to be better suited to the job in the eyes of your interviewers than anybody else. More than in most other professions, good references are important. Academia is a small world and people from different institutions often collaborate or meet at conferences. Consequently they tend to tell the truth in references.