Is it ethical for me to hire research assistant as a part-time PhD to help with secondary data collection?
In terms of academic honesty, there is in principle nothing unethical about using data that was collected by someone else, subject to all involved parties agreeing to this of course. Collecting sufficiently large clinical datasets is a long and complicated process, and there are entire departments specifically created to manage such databases and provide data to researchers. Papers solely analysing and re-analysing such publicly available data are very common in epidemiology, bioinformatics, statistical genetics etc., so that definitely will not be an obstacle to publish.
However, your supervisor and thesis committee may have different expectations about the work that you will do and skills that you will obtain during your PhD. Some issues that might come up are:

Are you outsourcing something that you are expected to learn during your PhD? Is the &quot;database use and organization&quot; something very menial, or too computational to be relevant for your field? Otherwise maybe you can do a part the process by yourself, and thus show that you have acquired enough knowledge of it?
Usually there are some regulations on how much time &amp; effort you should put into full time studies. If you do not do the data collection, will you be spending the same amount of time on some other academic work, beneficial to the project/lab?
Is there sufficient scientific advance/intellectual contribution/novelty remaining in your thesis, when viewed separately from the data collection? This one is the most difficult to judge. In the eyes of thesis examiners, doing some &quot;menial&quot; work often pushes the candidate towards bench biology, and this takes away some scrutiny from other parts of the work (even though it shouldn't). For example, you might be expected to have a deeper understanding of your statistical models, have some sensitivity analyses, maybe comparison between different models or such.

As long as all the involved parties agree on these expectations, I don't see anything unethical about this path.
Now, whether you are legally allowed to use the new grant for this purpose is an entirely different question, and will be determined by the fund's conditions and your institution's hiring policies.