How complicated are we talking?
It's good to hear you see some positive moves working with DNN and I'm sure you'll enjoy the experience.
The templates - are they single pages or mini sites? Often it takes a while just to get the initial 'master pages' right - it's not quite the right terminology.
The content - it's controlled and applied into the skin using panes - if you think rectangles and squares, then you'll get an idea on where the content is dropped in.
I think the easiest way would be to look at the template you are currently using and work out which modules you need - eg.. links on left hand side, announcements across the top, feeds to the right - .. whatever - you need to work out what modules you need to use on the site first.
the skin design often reflects the pane layout - but I think some designs have far too many panes - administration is bulky- so the ideal thing would be to have just what you need, remembering you can stack modules on top of each other.
You can't apply 'non dnn templates' in the same sort of fashion - but I guess you could copy the code into one html box - but really that would be defeating the purpose of using the product really. What you need to do is work out how to lay the content out in DNN and get the page just right, and then you can use a couple of features of dnn - if it's a page within the site you can use the 'export page' option, and store it in a folder for later use, or you can use the 'copy page' function where you can choose to use existing content, mirror the content, or keep the layout the same and build out the next page, but if you want to use in a 'mini portal' option, eg.. have a separate install under the same build - you'll need to use the 'export portal' option, under the host/portals option, and it is saved as a 'master layout' you can reuse within the site.
I know this is quite a few ideas here that are not perhaps complely articulated so well as it's hard to wrap up a whole set of ideas in one forum post, but what you want to do is completey doable in a dynamic sense, and you may need some more advice on how to get the structure in place.
I have seen several complicated layouts that work well in maintaining integrity and look and feel, and you can control it further by allowing permission to change content at the page or module level, or embedding specific information into the skins.
I hope this gives you something to make you a little more armed and dangerous.
Nina