You will have to have software that supports multiple video layers (eg Premiere, MediaStudio Pro etc).
Step 1
Film your office footage (no doors etc) as appropriate
Step 2
Film, or use a still image, the keyhole. It is extremely important that the keyhole is a solid colour (Black would probably do). For the effect to work smoothly the Keyhole itself should be in the center of the shot.
Step 3
Place your office footage on Video track 1
Step 4
Place your keyhole film, or still image, on the Video Overlay (V1) Track, adjusting the length of the shot to your requirements.
Step 5
Apply a moving path to the video, or still, on the video overlay track, that effectively enlarges the shot, so that the shot is eventually all solid colour.
Typically this is achieved by making the size of the shot in the end key frame about 3 times the size of the original (although you may have to play about with the size a bit).
Step 6
Render your movie and enjoy
The screenshots below (from MediaStudioPro) give a better idea of what is happening (Please do excuse the quality of the hand drawn keyhole, but any port in a storm etc)
Enjoy