Yes, that is possible!
How you can do it depends on with method you are using for displaying the subtitles. Usually that is one of these three methods: Media Player Classic internal renderer, DirectVobSub, or ffdshow. If you do not know which method you are using, then read the other FAQ topics on this page for more information about them. Below you can find the settings for all three methods.
Media Player Classic internal subtitle renderer
The internal subtitle renderer of MPC works differently from the other subtitle filters. Instead of merging the subtitle image with the video before it gets send to the video renderer, it sends the subtitle image to the video renderer separately from the video. The video renderer is then responsible for merging it with the video. The disadvantage of this method is that it only works with certain video renderers. The advantage is that the subtitles can be placed anywhere on the visible screen. There is no need to enlarge the video frame.
Subtitle placement options can be found here:
MPC Options -> Subtitles -> "Override placement"
MPC Options -> Subtitles -> Default Style -> "Screen Alignment & margins"
If you uncheck (or half-check) the option "Position subtitles relative to the video frame", then the subtitles are positioned relative to your screen (= video plus black bars). Positioning them relative to the screen thus allows placing the subtitles in the black bars that you see during full screen playback.
It is usually enough to adjust the positioning to be relative to the screen. The black bars are almost always high enough to fit the subtitles with the default vertical placement of 90%.
To use the internal subtitle filter:
MPC Options -> Playback -> Output > Subtitle Renderer
(For old versions: MPC Options -> Playback -> enable "auto-load subtitles")
To select a compatible video renderer:
MPC Options -> Playback -> Output -> DirectShow video
DirectVobSub and ffdshow
With these two subtitle display methods, the subtitles are merged on top of the video frame. Placing the subtitles below the video is only possible by enlarging the video frame. This is done by adding black bars to the top and bottom of the video. These black bars will also be visible when the video is not in fullscreen. They are different from black bars that are only visible during fullscreen playback. Those are not part of the video frame and thus can not be used to put the subtitles onto.
So two steps need to be taken: (1) enlarge the video frame, and (2) move the subtitles down a bit. Step 1 is the most important, step 2 is often not even needed.
Below you will find instructions for configuring DirectVobSub and ffdshow. You only need to configure the filter that is responsible for displaying the subtitles. Usually that will be DirectVobSub, unless you explicitly configured ffdshow to handle the subtitles.
(1) DirectVobSub options -> General -> Vertical Padding -> Extend to 16:9
(2) DirectVobSub options -> Main -> Override placement
(1) ffdshow video decoder configuration -> 'Subtitle' page -> Enable 'Letterbox' with value 16:9
(2) ffdshow video decoder configuration -> 'Subtitle' page -> Vertical Position
Values that you should use for DirectVobSub and ffdshow
Extend and Letterbox 16:9 means that the video height will be enlarged so that the ratio of the video width/height becomes 16/9. A different ratio may be more optimal depending on your monitor/tv. Some indications:
Monitor with screen resolution 1600x1280, 1280x1024, 1280x800 -> use 16:9
Monitor with screen resolution 1600x1200, 1024x768, 800x600 -> use 4:3
HDTV -> use 16:9
PAL TV -> use extend to 576 lines
NTSC TV -> use extend to 480 lines
Both filters have a default vertical position of 90%. To move the subtitles further down, increase this percentage with a few %. Don't make it too big, otherwise the bottom part of the subtitles may fall outside of the visible video area.