Such a tool probably doesn't exist because of some insurmountable physics problems. Sounds that have perceivable pitches are almost always made up of multiple sound waves. For example, if the note you're playing is an A at 440 hz, there will also be sound waves with these frequencies playing:
- 880 hz (2 * 440)
- 1320 hz (3 * 440)
- 1760 hz (4 * 440)
- 2200 hz (5 * 440)
... and so on, oftentimes indefinitely (although higher pitches generally get quieter, until they are impossible to hear). The pitches above 440 hz are called overtones. Different instrument sounds are created by changing how loud different overtones are and whether they're perfectly in tune. (Effects processors that don't overtly change overtones always do so as a side-effect.) http://en.m.wikipedia.org/wiki/Overtones
The problem with a frequency compressor is that it would through off these proportions. As a result, the instrument world likely sound like it no longer was playing pitches! In extreme cases, it would have a sound similar to an untuned percussion instrument.
For example, if you compressed a sound that started at A 440 hz and had noticeable overtones up to 2200 hz, then compress it to 1000 - 1200 hz, the overtone pattern would change to
- 1000 hz
- 1040 hz (1000 * 1.04)
- 1080 hz (1000 * 1.08)
- 1120 hz (1000 * 1.12)
- 1160 hz (1000 * 1.16)
- 1200 hz (1000 * 1.20)
These pitches are too close together to create a sense of pitch.