Yes, with certain firmwares you can. This adjustment is like turning up your stereo: it might get a bit "louder" but beyond a certain point it begins to clip and distort. The big difference is that unlike a stereo, the default radio gain is usually set fairly high -- there's rarely much more headroom to get.

A radio tuned for 100mw might have 200mw in it, if you're lucky. While this sounds like a lot, it's only 3db, which is a small fraction of any signal meter "bar" reading.

If you really want to increase your effective transmission power, your best bet is a high-gain antenna:

http://tinyurl.com/client-antennas (no affiliation)

These will increase your effective signal strength by focusing the beam of the antenna in the direction you wish to transmit/receive it. The down side is that the antennas have to be pointed at each other, and the higher the gain the closer they have to be aligned.

However, please keep in mind that increasing your effective radiation in one direction above the initial design limit is likely to run afoul of your government's (the FCC in the USA) rules about RF emissions.

In summary, yes you can do it, but it will take expensive, fidgety antennas, is probably illegal, and at the end of it you still might not get the amount of gain you need. I recognize your desire to use wireless but generally IMO it's not worth the work.