I think it's interesting that I was originally told that no one cares aobut this yet I'm getting a lot of help.
There's a difference between "We're interested in adding this functionality" and "We're interested in solving the problem of how to add it". It's an interesting mechanical issue. Try this:
Create a file called material_template_excrement.txt and add this to it:
material_template_excrement
[OBJECT:MATERIAL_TEMPLATE]
[MATERIAL_TEMPLATE:EXCREMENT_TEMPLATE]
[STATE_COLOR:ALL_SOLID:brown]
[STATE_NAME:ALL_SOLID:frozen excrement]
[STATE_ADJ:ALL_SOLID:frozen fecal]
[STATE_COLOR:LIQUID:BROWN]
[STATE_NAME:LIQUID:excrement]
[STATE_ADJ:LIQUID:fecal]
[STATE_COLOR:GAS:BROWN]
[STATE_NAME:GAS:n/a]
[STATE_ADJ:GAS:n/a]
[DISPLAY_COLOR:6:0:1]
[MATERIAL_VALUE:1]
[SPEC_HEAT:4181]
[IGNITE_POINT:10338]
[MELTING_POINT:10000]
[BOILING_POINT:10180]
[HEATDAM_POINT:NONE]
[COLDDAM_POINT:NONE]
[MAT_FIXED_TEMP:NONE]
[SOLID_DENSITY:500]
[LIQUID_DENSITY:1000]
[MOLAR_MASS:NONE]
[IMPACT_YIELD:10000]
[IMPACT_FRACTURE:10000]
[IMPACT_STRAIN_AT_YIELD:100]
[COMPRESSIVE_YIELD:10000]
[COMPRESSIVE_FRACTURE:10000]
[COMPRESSIVE_STRAIN_AT_YIELD:100]
[TENSILE_YIELD:10000]
[TENSILE_FRACTURE:10000]
[TENSILE_STRAIN_AT_YIELD:100]
[TORSION_YIELD:10000]
[TORSION_FRACTURE:10000]
[TORSION_STRAIN_AT_YIELD:100]
[SHEAR_YIELD:6600] used high salinity ice
[SHEAR_FRACTURE:6600]
[SHEAR_STRAIN_AT_YIELD:100]
[BENDING_YIELD:10000]
[BENDING_FRACTURE:10000]
[BENDING_STRAIN_AT_YIELD:100]
[MAX_EDGE:500]
[ABSORPTION:100]
[ROTS]
[GENERATES_MIASMA]
[LIQUID_MISC_CREATURE]
And add this to dwarves in creature_standard.txt:
[USE_MATERIAL_TEMPLATE:EXCREMENT:EXCREMENT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen dwarf excrement]
[STATE_ADJ:ALL_SOLID:frozen dwarf excrement]
[STATE_NAME:LIQUID:dwarf excrement]
[STATE_ADJ:LIQUID:dwarf excrement]
[STATE_NAME:GAS:boiling dwarf excrement]
[STATE_ADJ:GAS:boiling dwarf excrement]
[PREFIX:NONE]
[LAYS_UNUSUAL_EGGS:LIQUID_MISC:NO_SUBTYPE:LOCAL_CREATURE_MAT:EXCREMENT]
Try that out and see if it works. Your dwarves will want to lay a "dwarf egg" every three months or so, and will do so if a nest box is available, and because it's a liquid they won't sit on it, or be able to stockpile it, so it should eventually rot. If it doesn't work, look in errorlog.txt in the main dwarf folder and see what it's moaning about.
One possible issue is that it might not rot, as it would count as a contaminant. Blood and ichor are supposed to rot, but don't, so you might have to use a solid material's melting point and just accept that dwarves do it inside and refuse haulers take it away. You could even put in a reaction to convert dwarven excrement to potash so you can use it as fertiliser.