My question is: is it possible to change a room into another type of room? let's say I build a workroom above the kitchen, can I later turn it into a bathroom? (the workroom, not the kitchen)

Most of the rooms in VF3 have different options available, and you can see them all by tapping Store, then Home Improvement, then the particular room or area that you're interested in. You can change a particular room to any of the available options, but some room upgrades are locked until your little family has reached a certain generation.

The room you mentioned (which is actually two rooms) can have a bathroom in the room on the left and a workshop in the room on the right.

Yes, it would be nice to be able to add on some additional rooms - apart from what is already there in the structure.
I would like to put in the sauna, but there would have been nowhere to put a family room with pool table, play kitchen, bookshelves, couches, TV, etc. The living room (room off of front door)is too small for that, and nobody wants to enter someone's house in the informal setting of a den, where people just hang out.
It would be neat if on the back deck, we had an option of building a little sauna room.
Also, it would be really nice if they would allow us to move that big tent so we could have more room in the back yard! A lot of players have commented on that, but nothing happens.