A uniform standard would be the best I think.
online conversion of all kinds of units
I had a look at this and it doesn't seem to convert my own imaginary units(Hey, there is nothing wrong with imagining units...)...
Converting at all would probably invalidate all previous data files, and possibly saved games/maps too, if it was me I wouldn't want the risk unless it was tacked onto an already major update. if it was changed then there are two options, use a familiar measurment, or use a D.F. measurement. As for the familiar ones, Fahrenheit is only advantageous to those who are familiar with it, Celsius is only advantageous to those who are familiar with it and those who want to manipulate water, and Kelvin is only advantageous to those who don't want negatives, or want to deal with 0 Kelvin.
A system developed specifically for dwarf fortress could be very useful, it is a fictional environment which can have it's own laws. The melting point of magma could be lower then the melting point of water, water could be a liquid when cold, then as heat is applied it could turn into a gas, and then a solid, and all of this could occur at whatever arbitrary temperature is chosen. The more that Dwarf Fortress differs from familiar conditions the greater the extent to which a familiar temperature scale will be misleading.
So What are the options:
Fahrenheit: Good if you are familiar with it, and it seems that Toady is, and Toady has earned the right to use whatever system Toady wants...
Celsius: Well, it seems to be popular amongst humans, and water probably has the greatest combination of temperature-sensitivity and relevance to the game...
Kelvin: With magical heat sinks the whole thing about not worrying about negatives is gone, so unless some significance is implemented for absolute zero, it isn't really all that appetising.
Pick an arbitrary temperature as 0 and use the same units as Fahrenheit: Useful if you are familiar with Fahrenheit and like some element of the game as an anchor. But changing the 0 point messes up all the familiar values...
The same seems true when applied to Celsius and Kelvin.
Picking 2 significant temperatures within D.F. and picking arbitrary numbers to represent them, then plotting a direct progression between them to develop a scale: Could take alot of getting used to, but knowing that water freezes at 0 and magma will always be at 1000 could be useful...
Pseudo Kelvin, setting 0 to the point at which getting colder stops having any significant effect, and then importing a scale: No negatives is nice, but if you can't handle negatives then you probably won't be doing much modding, and most useful values will be quite obscure...
Blue Monkey! 666 = the temperature at which the sky turns yellow with purple polka-dots, 3 = the sound of the ocean as it turns into meatloaf, the scale is generated by progressing negatively from 9, alternating between a logarithmic and exponential progression, multiplying by 12 every christmas, until it wraps around and reaches 16: This is useful if you are making an artefact, or if they are watching you, or if your -carp leather loincloth- was too heavy to wear any more...