Of course you can - this is done all the time.
One of the nice things you can do with a built in is put a removable grill into the built in and put the speakers behind. You typically will want to have a nook for the speakers to fit into that is larger than the speakers so they can be removed.
When the speaker goes in, you want to fill any additional space around the speaker with foam to help eliminate sound reflections off the enclosure. The speaker grill can be removed and the speakers can be placed right up against your false front with acoustically transparent material over them.
This is done all the time in homes and high end home theaters and sounds fantastic when the proper precautions are taken. It is asthetically pleasing and gets the monster speaker out of your living area.
Of course - if the room is set up strictly as a listening room and is not a visible part of your living area, then why not leave them out? But, I am guessing that is not your situation.
