Okay, I'm stumped. What the heck is a 35mm adapter for the XH-A1? Are we talking 35mm film or 35mm lens, aburke09? One refers to the width of the film stock, the other to the length of a prime lens.
If you mean a screw-on wide-angle adapter for the A1, I fail to see how that will make your footage look like 35mm film. The A1 already has a pretty wide angle setting on it's zoom. Adding even more would distort the image, which I suppose is fine if you're going for that early Peter Jackson/Sam Raimi look, but it hardly screams "35mm film".
A good lens is a good lens. If you can afford a camera with a universal lens mount and a set of good quality prime lenses to go with it, then that obviously is the way to go. You'll nearly always get a little bit crisper image with a prime versus a zoom. Just make sure you have enough left over to get everything else you need, like directorik listed above.
That isn't going to make it look like 35mm film, though. In fact, nothing will make video footage look like 35mm film, IMHO. It's like painting an orange the color red in order to make it look like an apple; it doesn't look like an apple, just an orange painted red. The best you can hope for is to make the video look less like video.
Technically, there are a number of steps you can take to do this:
1. Light and shoot "film-style". If you don't know what this means I can't explain it here, but I'm sure there are plenty of books/web sites about it.
2. Record your footage in HD at 24p (or 24f with the XH-A1). This introduces a 2:3 pulldown which mimics the 24fps film rate to 30fps video standard used when film is transferred to video. You're actually lowering the quality of the footage in order to give the impression of higher quality -- ironic, eh?
3. In post, increase your contrast and decrease your saturation.
Again, will this look like 35mm film? Nope. But it will look less like video.