It’s possible…. but depending on how the layout is, it’s a roayl pain. But it can be done.
Eeeew!! Ick! I just looked at the source…. tables…. yuck.
In order to really be successful at porting it over you will need to be really comfortable at coding HTML & some PHP by HAND….
The way I would tackle it (And I’ve been successfull at getting this to work – my current design is a port) is to start with nothing. Start from scratch. Then add in div tags… one for the blog header, one for the page, one for the side bar, one for the posts, one for individual posts, and one for the footer. — I give each one a border of a different color so I can tell them apart. Then I play with the positioning until they are close to being in the right spot. In comes the sample text so I can see what it looks like, then I play with the colors, turn off the borders, adjust positioning, conver it into a php file, and start adding the wp template tags, making adjustments in the CSS to prevent it from going all to crap on me as I go along. Eventualy it comes out right somehow.
In your case, depending on how much HTML & CSS you know, you could probably take your existing layout, swap tables for div tags & some ul/li tags and adjust a few things and it would be just fine.
Tg