Get a tablet. For any illustrative based maps it really is a must. I'd never used one before and got one a couple of months ago because I got the chance to start working from home as an illustrator and so managed to get myself an A3 size Wacom Intuos4 XL and it's amazing how much better it makes my art. If you can put together some cash to get even a cheap one where it recognizes pen pressure then you will get instant results from your maps.

So to directly answer your question, no you can't get different thicknesses without a pen-pressure tablet (well there are certain calligraphic brushes that can give a similar effect but it would be very awkward to use them in this style).