Can you cut a fiberglass tub surround around a window in a tub/shower?

http://www.contractortalk.com/f74/tub-surround-window-80402/

You can cut it, but need to observe precautions when you do that. Where the edge cut is needs to be flush with the window sill. After cutting it a segment to cover the window sill will be cut. It needs to fit in very snuggly. When the window sill and side have been fit, glue them down. Then the entire window encasement needs to be sealed with an epoxy sealer. If you do not seal it, water will get behind the surround and will rot out the wood and walls behind it.

My contractor did, I have a large really old window right in the middle. Worked out great.

Some people with windows in the shower have a shower curtain just for the window. When they use the shower the pull the curtain shut. Keeps water off the window and you have more privacy.

No. Unless you know what you're doing. There's way too much work and water proofing for a person to do himself/herself unless you really really know what you are doing.