There's really no such thing as a "Dreamweaver website" or "Dreamweaver code", it's just html, css, javascript etc (very unlike Adobe Muse).
There would be nothing to "change over" to DW. All you really need to do is create a Site Definition in DW using your existing local site's root folder. You would then be able to work with your existing pages within DW without issue.
As for making templates, There are two kinds of templates when speaking about DW. "Design templates", and "Dreamweaver .DWT Templates". A design template is just a page you ultimately use Save As to create the pages of your site from, nothing fancy. A .DWT Template is a file used only within DW itself for creating Child pages whose design relies on the .DWT Template file. When you change non-editable regions of a .DWT locally, DW will update all Children of that Template. You then need to find and upload all of the changed Child pages to the server. The DWT itself has no affect when uploaded, so generally isn't.
I would suggest staying away from DW's .DWT Template system (if that's what you were asking about) if your site is ever going to be larger than maybe 50 pages at any point in the future. They're essentially the "poor man's server-side include replacement". If you learn how to work with simple .php or .shtml includes instead, you're going to have a much easier time with site-wide updates than you would with .DWT Templates. With server-side includes, you modify the include file, then upload it to the server, the server then updates the pages. There's no need to figure out which pages get the update and upload them individually, it's all handled automatically by the hosting server itself.
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