I'm a new designer to Marketo, but I've used other CRM's like Watson™ Campaign Automation (Silverpop). I've always been able to start/create the email code in Dreamweaver and package a zip of the html and images to upload into the email sending program. We have a bunch of emails built in dreamweaver, but I'm not seeing an option to upload those into our new Marketo account. I'm not sure the best practice to get our emails working in the Marketo system.
If you navigate to Design Studio and then the Emails section and finally Templates, you are able to create a new template using the code from Dreamweaver. Another solution for one-off type emails is when you edit the email, there is an "Edit Code" but in the top-middle of the editor screen where you can drop in the HTML. Hope this helps.
There is some documentation you'll want to refer to here (bookmark worthy): Email Template Syntax - Marketo Docs - Product Documentation
In Marketo, the email template world is divided into 1.0 and 2.0 environments. You may need to activate EM 2.0 to take advantage of the newest editor (highly recommended), here's a little more info on that: Turning On/Off Email Editor 2.0 - Marketo Docs - Product Documentation
There is some nuance to be aware of in the syntax you'll need to use to get things working from Dreamweaver into Marketo. The key differences between a 'raw' HTML template (from Dreamweaver, platform agnostic) and a Marketo HTML template is calling out the editable areas. In the case of email 2.0, there's also a layer of variables available to deploy via <meta> tags in the head.
A word of caution on using the "edit code" functionality from within the editor... (and there's a warning at the top of that page to this effect)... editing the code at the page-level will disassociate it (break it) from the template it's tied to. For the sake of one-off or quick stuff, this can be a handy stop-gap, but in the email 2.0 environment, breaking your HTML (email) from the template means you'll lose the modular functionality it offers. I had to adjust a bit to editing HTML in Marketo when I started because I was used to the "load it all once" process - even for editing.
To set up a new template, you should be able to create a new email template file which will be pre-populated with some basic Marketo syntax. Maybe have a look thru the default Marketo syntax on the starter to get familiar or even borrow some of it to cobble into your HTML from Dreamweaver. From there, if you want the end-user to be able to edit the email inside the editor (rather than via an HTML-only [Dw] in/out) you'll need to add "editable areas" (element class="mktoText" or class="mktEditable") around the sections you want the end user to be able to change. This allows you to "hide" some code from being edited and "expose" other code to be manipulated by the end user. If you're not planning on doing your editing in Marketo, none of this may matter - you can just keep replacing the code as you update on the template or Email asset w/ the "Edit Code" dialog.
It's well worth setting up the email 2.0 modules for their drag-and-drop'ability, it really allows you to recreate and adjust an email (all the way down to font size and colors via variables) without needing to adjust the HTML (for most things). I've found this to be worth the time in the long-run, but depending the size and cadence of your organization, this might be overkill. i.e. If email is already something you do without much effort, keep-on-keepin' on, but if it's something that's a hassle or complex this could save some time downstream by adding flexibility into the design (that's the main advantage of 2.0 vs. 1.0 is the ability to add and remove sections of content (clone, duplicate, add, remove) easily and on-demand.
If you run into any issues adding the syntax to get something "live" I'd be happy to have a look at the code and help get that running with you.
Good luck and happy coding!