Ubuntu :: Mount Into A Non Empty Folder / Achieve This?

I need to do the following things and googling didn't help much:

1.mount a volume(or windows share, whatever) into a non empty folder
2.both the original content of the folder and the content of the newly mounted volume is accessible
3.updated and newly created files can be written to the mounted volume or the original folder determined by something like a switch at mount time
4.if files with same names exist both in the folder and on the volume, then which file to be presented to the OS is determined by their last modified date/time.

any one can give a walk-through on how to achieve this?