Dear,
I have gone through the work requirements and sure, it can be done but will you please ping me here on chat for some confirmation.
Here,
You will get access to about 3 CMS system & you will use the pages from them to create the new CMS
You need to be able to make small HTML/CSS additions & adjustments & keep everything responsive & don't lose the existing PHP code on the pages
> Do you have access of PHP site? I'm sure, the HTML/CSS can be done and adjustment with the site and responsive will be exact what it was before.
I look forward to discuss you soon.
Regards
Bhuwan Roka